The tickrate is the amount of times the server "ticks", or carries out "think" operations, per second. This includes hit detection, physics simulation, and any addons that hook the "Think" event.
Higher tickrates use more system resources, but also grant a greater, better, snappier, more responsive gameplay experience. Lower tickrates use less system resources but make the gameplay very choppy and unresponsive.
I usually use 66 on mine, as it's a great balance and is default on NFOServer brand hosted servers because their machines are heavily underloaded.