Bookmarklets are small Javascript scripts that can be executed on a page by clicking them on the bookmarks bar. They provide a way to extend the capabilities of the browser in a light-weight way compared to building and publishing a browser extension. They don’t require installation or review process so building and sharing them is easier.
Warning
Copying other people’s bookmarklets can cause a security problem. Be careful and inspect the code to your best ability or find someone who can review them before installing.