Bookmarklets are kind of a cross between bookmarks and browser extension - they don't need to be installed; they look just like bookmarks on your bookmarks bar, but instead of loading a page when you click them, they do something to/with the current page.
How to Install a Bookmarklet1) Drag the link to your bookmarks bar.
(Alternatively, you can right click on your browser toolbar, and click "New Bookmark" (in Firefox) or "Add Page" (in Chrome), and paste the link in the "location/URL box.
2) Give it any Name you want (eg: Download Youtube)
3) Save it
How to use a Bookmarklet Whenever you're on a the page that you want to use it, simply click on that bookmark.