What is "bookmarklet"

Bookmarklets are "one-click" tools that add functionality to the browser, such as modifying the appearance of a web page, query a search engine with search terms provided by previously selected text, or send the current page to a translation or blog service.

Are small programs similar to Extensions, but simpler, with less functionality

https://support.mozilla.org/pt-BR/kb/bookmarklets-realizar-tarefas-comuns-pagina-web