There are 9 Energy Centers depicted in a body graph in Human Design. They are mirrored after the chakras and tree of life. They are the Head, Ajna, Throat, Will/Heart, Sacral, Emotional Solar Plexus, Splenic, G/Identity, and Root Centers.
These Energy Centers can either be defined (colored in) or undefined (white). What makes a Center defined is if there is a defined channel leading from one Center to another. To have a defined channel, you need 2 defined Gates. The Planets' placements determine which Gates are defined.
Anything that is defined is fixed/set in stone. It is highly resistant to change. The more Centers you have defined in your body graph, the less other people are able to influence you. Obviously context matters here. But defined = rigid/fixed, that's why I say that. It is something that will always be present in your life and who you are. It's consistent/fixed.
Anything that is undefined is variable. You experience it in a variety of different ways. It is not fixed nor set in stone. It is inconsistent/variable. You experience this energy in yourself and your life randomly. Whereas with something defined, you experience that in a very consistent manner.
You can think of anything defined as a fixed movement of energy. It moves in a very set way/pattern. You can think of anything undefined as a variable movement of energy. Energy is moved at random. It operates at random. It has random ways/patterns. As I already said, you experience it in a variety of different ways.
Any Undefined Center is a place where you are most vulnerable to conditioning from your environment/other people. Any defined Center is a place of consistent energy within you. It's like a "fixed" energy that is harder for the environment/others to tamper with.
So let's say you have a defined G/Identity Center. You are more inclined to basically know who you are and what you identify with. Someone with an undefined one would be more inclined to be not sure who they are and have their identity more easily conditioned by outside influence.
The Identity Center is all about identity & direction. It is home to what is called the "Magnetic Monopole". To put this very simply, it means it attracts things towards you. It attracts your path/destiny/identity into your life whether or not you have it defined. Whatever it touches, it colors your identity/path with it.
Of the 9 Energy Centers, 2 are what Human Design calls "Pressure Centers". These are the Root and Head Centers. They are called Pressure Centers because they basically exert pressure that then spreads out to the other Centers. It's like a circuit.
The Head Center provides mental pressure. It is like a desire to know or understand. It's the desire or compulsion before an idea itself is presented. It makes you want to ask questions, it makes you want to figure shit out. It pressures you to start a thought process. It can come out as mental anxiety, scrambling thoughts, etc.
The Root Center provides adrenaline or a sort of physical pressure. It is connected to the Sacral, Solar Plexus, and Splenic Centers. So it pressures you to survive (Splenic), to create/generate (Sacral), and to feel (Solar Plexus). The Root Center provides the production of stress hormones. It basically gives you pressure to be FREE. Its energy operates with pulses. It also happens to be one of the 4 Motor Centers as well. A Motor basically provides you with energy to manifest and do things.
There's 4 Motor Centers. The Root, Sacral, Will/Heart, and Emotional Solar Plexus Centers. These provide a certain type of energy you use to manifest and do things.
Root would be adrenaline energy that has on/off cycles
Sacral would be sustainable/generative energy
Will/Heart provides the ability to manage and create material resources for short periods of time (kind of like the cycle of on/off with the Root)
The Emotional Solar Plexus Center provides emotional energy to use
The Emotional Solar Plexus Center also happens to be an Awareness Center along with the Ajna and Splenic Centers. It provides emotional awareness. And its energy works in waves. There's 3 different types of Solar waves, but I'm not going to cover that right now.
The Solar basically houses Spirit Consciousness & emotional intelligence. The Ajna is the home of Mind Consciousness, mental concepts, and mental awareness. The Ajna is kinda where your thoughts are. The Head Center provides the Ajna with fuel/questions to conceptualize/answer.
And then you have the Splenic Center! It is home to Body Consciousness and survival instincts/intuition. It brings instinctual awareness. It works instantaneously. Someone with a defined Splenic Center will have instantaneous knowings about things. The Spleen only speaks once though! It will not tell you what's good again, it will only ever do it in the moment. This Center is rooted in survival and bodily functioning. It rules the immune system and also instinctive fears.
Last but not least is the Throat Center! This is your Center of manifestation and expression. This is where all energies in the body graph try to exit. The energy of all the centers are trying to make their way here. If you have a defined Throat Center, you have a fixed method of expression and manifestation. If you have an undefined Throat Center, you have a inconsistent/variable method of expression and manifestation.
And that's your quick overview of the 9 Centers! Be sure to check out my Ebook(s) to get more information!