Popups, I do not like them, at all. While they are sometimes used by sites to make part of the website's functionality available, they are more often than not used for advertisement. And while there is nothing wrong with advertising in general, popup ads tend to be one of the most annoying forms of advertisement that you encounter on the Internet. The core reason being that new windows are spawned, that you need to close manually at one point or the other. That's fine if one window spawns, but there are sites out there that bombard you with popups.
The extension comes with two modes. First whitelist mode, which is the default mode after installation, and then blacklist mode. Whitelist basically blocks all popups automatically, except on pages that you have explicitly whitelisted in the options of the extension. Blacklist mode on the other hand turns this around by only blocking popup windows on pages that you have added to the list. The latter may be useful if you visit that annoying one site regularly that is spawning popups like crazy.
Blocked popups are indicated in the browser's address bar, next to the bookmarking star. Depending on the page you are on, you may see icons that indicate a blacklisted or whitelisted page, and a blocked popup.
A click on the blocked popup icon in the address bar displayed information about the page the site tried to spawn, with options to open it by clicking on the link, or by going to the blocking preferences of the extension to add it to the whitelist or blacklist, depending on your configuration.