A transistor is an electrical component that functions, most basically, as a switch — in principle not so different from a light switch. Instead of a physical movement, however, a transistor is controlled by a flow of electricity. And unlike your basic light switch, a transistor can be on, off, or somewhere in between.
Most transistors have three connections: one for current in, one for current out, and one that controls the “switch.” The current flowing through a transistor can be larger than the current controlling it, so it can become an amplifier: Connect the input to a power source (like a battery), and the control lead to a weak signal (like a guitar pickup), and the output will sound like the control signal, only louder. Just how much louder depends on a lot of things, but a factor of 200 is routine. This number is called gain.