Imagine standing in an open field with a bucket of water balloons and a couple of friends. You've decided to play a game called "Mind." Each of you has your own set of rules. Maybe Molly will throw a water balloon at Bob whenever you throw a water balloon at Molly. Maybe Bob will splash both of you whenever he goes five minutes without getting hit -- or if it gets too warm out or if it's seven o'clock or if he's in a bad mood that day. The details don't matter.
That game would look a lot like the way neurons, the cells that make up your brain and nerves, interact with one another. They sit around inside an ant or a bird or Stephen Hawking and follow a simple set of rules. Sometimes they send electrochemical signals to their neighbors. Sometimes they don't. No single neuron "understands" the whole system.