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We basically don't know a whole lot about the human brain.
We do know about the microscopic behavior of a single neuron though. Basically a neuron is a cell with multiple long "branches" that connect to other neurons. A neuron can get stimulated by getting a signal from one of the neurons connected to it. But that's not enough the signal has to be over a certain strength to make that neuron "fire" which means it itself gives a signal towards its connected neurons.
Neurons can also become "drained" where they stop reacting to signals for a little while or "supercharged" where they give bigger signals than normally for a short while.
On a slightly bigger scale we have "Neural nets" which are a couple of neurons usually between 1000-10,000 neurons that provide a specific function together and "transmit" the results of whatever they are doing to the next part of the brain. Modern neural net AIs are based upon this behavior of biological neurons.
If we go even bigger than that then you have "brain regions". Basically every brain has a region that is specialized for doing things. It has to do with what evolved first. So the brain stem which evolved first does things like reflexes, heart regulation, breathing. One above that is motor control. Then the layer on top of that is sensory input so vision,hearing,smell etc. Then on top of that is specialized sensory input such as sensitive touch in human fingers or specific pattern recognition in vision. Then on top of that is emotions the "mamalian brain". And finally the last piece on top of that is the prefrontal cortex which is the "rationality" part of the brain unique to humans and also hypothesized to house our consciousness.
More than this we actually don't know.
