When a creature you can see ends its turn, you can take a Reaction to freeze time for everyone but that creature while it takes an extra turn. No time passes for other creatures, including yourself if the target is another creature.
The target's extra turn ends early if one of the actions it takes, or any effects that it creates, affects a creature other than itself or an object being worn or carried by someone other than it.
Once you use this feature, you can't use it again until you finish a Long Rest.