A tick is the smallest amount of time the server does its "work" in. It does not mean, as the person I'm replying to was implying, that clients are 100% always lined up, because a game is played (locally) at much higher update rates.

So a server, of course, does send updates (player position, etc.) every "tick," but that doesn't matter. Even assuming zero dropped packets (suppose we're playing over TCP), it would feel like shit (stuttering, rubber-banding, pop-in, jittery physics, etc.) to play a game over a ~60ms latency that updates ~60 times a second vs other people that also are ~60ms from the server, so game engines do a lot of interpolation and servers are in charge of concensus. Hence, it's a bit of a misdirection to say: "can't you just send everyone the right player data every server update?" because servers obviously already do that (and that's not really the hard part, anyway).

Local interpolation and remote concensus is the hard part, and games handle this differently. In CS, for example, two players cannot kill each other (with guns) simultaneously. Valve's engine requires that someone always wins a gunfight (which sometimes can feel random). However, I would argue that feels way better than, e.g. in Halo, where you can headshot each other (and both players die), which feels dumb and frustrating.

So when building these servers, there are lot of tradeoffs to consider (a lot of which might change the feel of the game).