Using userscripts involves running code from the internet, which carries risks.
Tampermonkey is a "userscript manager." It doesn't block ads by itself. Instead, it runs small snippets of JavaScript code (scripts) that modify how a website behaves after it loads. adblock tampermonkey
Standard adblockers are great at blocking pre-roll ads on YouTube, but they can struggle with embedded players on third-party streaming sites. Tampermonkey scripts can modify the player size, remove the "close ad" countdown timers, and strip out the "Recommended Videos" suggestions that often clutter the screen. Using userscripts involves running code from the internet,