What Does Bookmarklet Mean?
A bookmarklet is a tiny program that is stored as a bookmarked URL in a user’s web browser. When clicked, a bookmarklet peforms a basic task, such as submitting a query to a search engine or changing the appearance/configuration of a page.
Bookmarklet combines the names and meaning of “bookmark” and “applet”. They are often just a few lines of code and most often written in JavaScript.
Techopedia Explains Bookmarklet
Essentially, a bookmarklet is just a URL that starts with “javascript:” instead of the standard URL “http://”. Because the browser knows that the protocol to be used is JavaScript, it treats every string after that as JavaScript code. When the code is executed, it will have access to the current page and is free to do what it is programmed for. It can inspect or change elements of the Web page and change settings like the font color without reloading the page.
This method of playing with the website is very safe as it is only executed on the current instance of the page the user sees – it does not change the source of the site.