UBlock Origin is a popular and efficient content blocker which can protect you from ads, trackers, malware and more.
If you've ever used another ad blocker you'll already understand the basics. UBlock Origin adds an icon to your address bar which shows the number of requests it's blocked; if everything's fine you can leave it to run entirely atomatically, but clicking the icon displays a panel giving more control.
If a site isn't working, you're able to toggle particular blocking types - popups, large media elements, remote fonts, more - with a click. An element picker enables fine-tuning blocking for a specific part of the current page, or you can turn blocking off entirely for the whole site.
UBlock Origin uses multiple sources to identify what to block, including Malware Domain List, EasyList and Peter Lowe's Ad and tracking server list. The "3rd-party filters" tab in uBlock's Settings shows you all the currently active filters and has many others, but treat this with care. The more you select, the more likely you are to have problems, without necessarily much gain in security.
Experts can customise the program with their own filters, and there are a few other potentially useful extras. If you're using a VPN which doesn't block WebRTC leaks, for instance, there's a Privacy option in Settings which should do it for you.
An excellent content blocker, highly configurable, packed with features, but also carefully optimised to be as lightweight and speedy as possible.