What an ad wall is
An ad wall is an interstitial page (or several) that a shortener shows before the real destination. It usually combines a countdown timer, banner and pop-up ads, and a button you can only click once the timer ends.
The goal is to keep you on the page long enough to load ads. The real link is there the whole time, the wall just delays it.
The typical steps
First a countdown runs while ads load. Then a 'continue' or 'get link' button appears. Often there is a second page that repeats the process, and sometimes a captcha to prove you are human.
Each step is another chance to show an ad. By the end you have spent twenty or thirty seconds and several clicks to reach a single address.
Getting past it safely
A resolver walks through these steps automatically and returns only the final destination. You see the same link without the ads or the wait, and nothing is installed on your device.
It is a convenience layer over a redirect the shortener already defines, not a hack, you simply reach the address the short link was always pointing to.