To sum everything up, TIMERS are just TICKS. They are the atomic unit time for a scene. It is the value of each TICK that is important and usually 1 TICK = 1 ACTION for players. Atomic means a tick cannot be divided down any further.
In combat, TIMERS are ticking each ROUND, which is the default assumption. You can still have TURN TIMERS for some important things, like DEATH TIMERS, which are ticking every TURN. After each player plays their turn (in this case, players won’t get an ACTION every TURN), the TIMER goes down.
Like many examples written above, you can say that each TICK of a TIMER is a certain length in fiction time (in-game time).
I say “Boys 'n girls, now each round is 10 mins long and each of you can take single action. In [ I roll 1D4 and the result is 2 ] 2 ROUNDS, this barrier in front of you is gonna collapse. Player A, what do you do?” In this example, 1 ACTION = 1 TICK = 10 mins.
Obviously players love to complain why in combat 1 TICK = 1 ROUND and while in exploration 1 TICK = 1 HOUR. I explain the difference simply like this: Combat is filled with adrenaline, quick thinking and quick reaction, whereas exploration is avoiding danger, finding a route, staying alive and whatnot. Naturally these things take a lot of time. Characters cannot always be in combat and therefore in ROUNDS because they don’t have such stamina (nobody has).