Definition - 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.
Techopedia explains Bookmarklet
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.