It all comes down to a redirect
A link shortener stores a short code that maps to a longer destination URL. When you open the short link, the shortener's server looks up that code and answers with a redirect to the real address.
In the simplest case this is instant. The complexity comes from everything the shortener layers on top of that single redirect to earn money from your visit.
You can think of the short code as an entry in a lookup table: 'AbCdE' points to a long URL, you open the short link, the server reads the table and sends you on. Plain shorteners, the kind used purely to make a long link tidy, do exactly this and nothing else, so they feel instant.
Not all shorteners are the same
There are two broad kinds. A plain shortener just makes a long address shorter and redirects you immediately. A monetized, or 'ad-gated', shortener adds a wait and ads on top, because its owner is paid for the time and the views.
The links people want to skip are almost always the second kind. The destination is identical to a plain link; what differs is everything the shortener piles in front of it.
Why the wait and the ads exist
Many shorteners pay their owners per visit, so they insert a countdown, a 'get link' button and one or more ad pages before they finally redirect you. The longer you stay and the more ads you load, the more they earn.
That is why opening a single short link can mean clicking through several pages, dismissing pop-ups and waiting out a timer before the real destination appears.
This is a business model, not a technical necessity. The countdown, the extra 'continue' pages and the pop-ups are all tuned to keep you on the page and load as many ads as possible before letting you through. The link was ready the whole time.
What a monetized shortener makes you do
A typical ad-gated link walks you through a sequence: an initial page with a timer, a 'get link' or 'continue' button that only wakes up when the timer ends, sometimes a 'confirm you are human' step, and one or more ad pages in between before the real address finally appears.
Each of those steps is a chance to show you another ad. That is why one short link can turn into several page loads.
Are shortened links safe to click?
A shortened link is only as safe as wherever it points, and the wrapping hides that until the very end. Most are harmless, but because the destination is out of view, short links are also a favourite way to disguise unwanted or malicious pages.
This is where seeing the real destination first helps: knowing where a link truly goes before you open it is safer than clicking blind through ad pages to find out.
How resolving skips the busywork
A resolver skips the ads and the wait for you and hands you the real address directly, instead of making you click through the intermediate pages yourself.
The destination is identical to what you would reach by clicking through manually; you simply skip the ads and the wait, and you get to see the final address up front. Paste a link on the supported sites page to watch it happen.