Simply put, many websites contain ads. And some of those ads can 'pop up' in their own window.
A blocker can prevent ad windows from opening, or close them when they show up.
They can also block some of the ads on the webpages you are looking at.

See also:

• https://support.mozilla.org/en-US/kb/pop-blocker-settings-exceptions-troubleshooting

Firefox shows a blueish highlighted shield icon at the left end of the location/address bar in case Tracking Protection is blocking content. You can click shield icon for more detail and possibly disable the protection. You can check in the Web Console what content is blocked.

• "3-bar" menu button or Tools -> Web Developer
• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

A simple question deserves a simple answer.

It's not as simple as you might imagine. All popup blockers stop popup windows from opening (and popup windows diverted to a new tab instead), but they have their own policies on when to make exceptions. After all, if you can't ever let a site's link open a new window or tab, you may find your browser less useful. Firefox's built-in popup blocker generally makes exceptions for user-initiated windows that follow a click, and there are some other exceptions, too.