What is a tick in a game?
In one sentence: One beat of a shared game world: the server updates positions and scores this many times a second.
A little more
In a shared online game the server updates the world on a steady beat. Each beat is a tick. At 20 ticks a second the shared park is refreshed every 50 milliseconds; at 60 ticks, every 16 milliseconds or so.
Between ticks the server queues messages from every player, then applies them in order and tells every tablet the new picture. Faster ticks feel snappier but cost more work on the server and more messages on the wire.
It's a bit like… A metronome for the shared playground: on each click the park jumps to its next moment.