Unfortunately, it doesn't guarantee that you'll be invited, but it does mean that should someone have to drop out or be withdrawn, that you will be good to go if you are selected off the waitlist.
In any group there are usually a few people who drop out for various reasons - medical, professional, personal, etc. There are others who won't be legally or medically cleared to go (which happened to me, so someone on the waitlist should have been given my slot). I've heard of cases where there were enough people who had to withdraw that the entire waitlist ended up staging. On the other hand, if no one drops out, no one from the waitlist will stage, and you will have to reapply.
If you DO have to reapply, they can reuse many of your submissions for your next time through, which puts you a leg up. Many of the medical and dental checks are good for 1-2 years depending on what it is, and legal clearance is good for 6 months. They can reassign your passport from your 'old' country to your new, which means you won't have to do that again, and many of the items in the New Volunteer Portal will already be filled out if you get re-invited and have to do it all again (I say this from experience, unfortunately).
EDIT: One quick suggestion - make sure you save copies of everything you upload to MAP. Even though they used them again, you will get a new case # when you reapply, and all your docs will be wiped from that # and have to be re-uploaded.