A browser plug-in is an application that runs inside your browser. When you install a plug-in, it’s like adding a small program to your computer. Usually, you have to add a plug-in when you visit a website that uses a plug-in. You can use this feature to view online content such as video and text.
A browser plug-in can be either open source or proprietary software. Open source is free to download and use while proprietary software is usually paid for by the user. Open source plug-ins are usually made by volunteers and can be downloaded from the Internet. Proprietary plug-ins are often created by companies and can only be obtained through special software distribution channels (such as CD or USB drives).