The brain consists of long nerve cells called neurons.  Each neuron has a long branch on one end that receives electrochemical signals from other neurons, a cell body in the middle, and a second long branch on the other end that passes electrochemical signals on to other neurons.  If the receiving end of a neuron receives enough input from other neurons, an electrochemical wave runs the length of the cell.  In that case, the neuron is said to be “firing.”  Each thought (or emotion or perception or initiation of a bodily action) corresponds to a particular set of firing neurons.