The game simulation is updated (recalculated) every 100ms (at speed 0). One such update is called a "tick". Only when a new tick is calculated can inputs be applied to the simulation. This means input is delayed by 5 ticks (500ms / 100ms). So the tick in which the input was made (and sent to others) is different from the tick in which it is applied to the game.
The F11 windows mostly tells you for how many past ticks a player's input has not arrived yet at YOUR computer. For example if you see that "player 2" is behind by 3 updates/ticks/packets that means that you havent yet received the input that you will need from that player 2 ticks in the future. When a player is behind 5/6 ticks, your game simulation stops, because you need his input (which he should have made and sent 5/6 ticks ago) RIGHT NOW.
The F11 window shows you only the connection between you and other players, not between two other players. (every player is connected to every other player), so everybody sees something different in the F11 window.
So when you see that one player is 6 ticks behind in sending you their input and game has stopped, but every other player is 1-2 ticks behind, that means:
- Maybe that players internet has a temporary problem (might reconnect though)
- Maybe that players computer/game has crashed so he not longer sends his input
- Maybe only the connection between you and that player has a problem (his connection to other players could still be fine, this is why its useful to ask others in chat if they see the same player being behind)
Sometimes a player is not constantly behind 6 ticks but is oscillating between about 4 and 6 ticks, resulting in a stuttering game, that means:
- Maybe ping spikes above 500ms regularly
- Maybe that players computer is so slow that the game cannot slow itself down enough to account for bad performance.
- Maybe that players computer has performance spikes (thermal throttling) that confuse the game enough to make it not slow down how it should
If one player is constantly sending his input but at a slower rate than other players, the game slows down (time between ticks is increased) until that player can keep up. Thats why game speed for everyone is determined by the slowest players computer (because the game simulation must be kept in sync on every players computer).
Keep in mind i don't know if the input delays is fixed to 500ms or to 5 ticks. If it is defined in ticks, that means it would change when you change game speed. Not sure. If it is defined in ms that means that the amount of ticks that somebody can be behind without causing the game to stutter/stop can vary based on game speed.
As FTx said, desyncs are a different problem.