A transformer is an electrical power component consisting of two coils of wire wrapped around an iron core.  A basic transformer has four wires: the two ends of each coil.  If additional wires are added to points within one of the coils, these are called taps.  A tap in the middle of a coil is a center tap.  If the center tap is used as a reference point, electric current at one end will be the opposite of the other end — when one is positive, the other is negative, and vice versa.