What does that mean and what is open admission?
If you are waitlisted for a college, when are you told if you’ll be accepted?
Rolling admission simply means first come, first serve basis. If you have decent stats and you apply early, you may get in while your friend who has better stats may not later on.
Every school has different policies on waitlist starting from whether they even offer waitlist to how many they pick from the waitlist. Look up Common Data Set for schools you are interested in.
Rolling admission only means they will start reviewing your application earlier if you do submit early and they will release admission results over a long period of time instead of a fix date or week. Early applicants would have a slight advantage as space would be limited near the end. Nevertheless, qualified students are likely to receive their admission earlier while marginal students may still need to wait for a while even they submit their application early.