A browser extension is a plugin for an internet browser that adds additional features.
A browser extension is a plugin for an internet browser that adds additional functionalities and features to it that were not originally there or developed by the developers of the browser itself. Extensions have the ability to modify the user interface and add Web service functionality to a browser.
You can think of a browser extension as a small unit of software, oftentimes referred to as a plug, that can perform various controls as well as filters to change the way a user can visit a web page or view information from a specific web service.
Alongside being used to enhance the functionality of a website, browser extensions can also be used to remove unwanted elements, such as pop-up advertisements and auto-play features in regard to online videos.