so in the brain we have two main type of neurons ( excitatory and inhibitory ) or glumate vs gaba. When glumate is release it causes a neuron to fire and thus exitatory. Adenosine is release when neurons fire sometimes (what we call presynaptic) and feed back onto itself, when it binds to the receptor it inhibit glutamate release and thus decreases activity. There are other mechnism, like I mentioned above such as increasing release of gaba, but I think this is the main one.