Time Stop is a Power in BTD6. It temporarily freezes the screen for 10 (?) seconds. All bloons are reduced to 10% of their original speed but all towers' attacking speeds are reduced to 30% of their original speed. Before the 11.0 update, everything on screen would stop moving (this includes bloons AND towers). However, both of these versions of Time Stop will offer the player additional breathing time in order to change defenses, allowing players to decrease stress levels while doing so. It can also be used to refresh cooldowns of towers when they're not available. It should be noted, however, that Time Stop will not affect BADs at all.
For some reason, the 11.0 change made the ability no longer affect B.A.D.s, meaning that using Time Stop against a B.A.D. actually makes the defense weaker. It is unknown if this is a bug or intentional, but this shows that Time Stop is probably uses a slowing similar to Bloon Sabotage.