I am a casual American football viewer but my understanding is that the kneel ends the current play but keeps the clock running. Each team has something like 40s to setup their formation and snap the ball after the previous play has ended. If the game clock is still running (this is concurrent to their 40s of "setup time"), the team that is in possession of the ball can just use the full setup time (idk the formal term for this) to just run out the game clock.

Each team has 4 attempts to move the ball forward 10 yards, where if the ball moves >= 10 yards they get a fresh set of 4 attempts. These are called "downs".

If the team has any downs left when they kneel then they can maintain possession of the ball and can thus run out the clock. Most (all?) of the time the teams end the game even if there is time left on the clock.

Note that either team can call a timeout pre-snap which freezes the game clock. Certain plays also result in the game clock freezing between plays. There is also a 2-minute warning at the end of the 2nd/4th quarter that also freezes the game clock.

IMO clock management adds a very interesting strategic layer to NFL football.