From my understanding, only duration is affected until after the 3rd stick of the effect (it's duration is at 1/4 normal time). After this point the monster becomes completely resistant till the timer is up, then it resets. To answer your question, the "*" means if the monster has an 25% resist rate on sleep, it will stay 25% till after the 3rd stick, then it's 100% until the timer runs out, at which point it returns to 25% and the cycle repeats.

Using an example, if you land sleep 3 times in 3 minutes, all attempts to land sleep will not work until the last 1:15 has passed (assuming it was slept right after it woke 3 times leaving 1:15ish left on the timer). Any attempt to land sleep while it's already asleep also gives a resist, so you can't stack sleeps and build resistance really fast. I assume it's the same with all the other CC effects but I never tested them. Unsure if same rules apply to your character now as they did stack at some point (like dodos could re-sleep you again and again and just resetting the timer with their breath move, haven't really payed attention to see if this has changed).