A game-tick or just "tick" is 1/60 of a Game-second.
Everything in Factorio is calculated with that base. For example the speed of anything is internally calculated in Tile/tick.
The real-world duration of a game-tick depends on the speed of your computer, and is at most 1/60th of a second (0,01667 seconds). A tick can last longer when the game stops (say, to autosave) or if the framerate drops below 60FPS.
What is done during one tick?
This FFF-article explains what the game engine calculates on every tick.