Vita plugins are small programs that run in the background of the operating system which adds various features and tweaks to the system. for example you will already have several plugins installed that is helping your modded system to run as it does.
There are quite a few plugins available and it can be difficult to know whats good and whats outdated so be careful with what you do choose to check out as you could potentially cause issues with incompatible plugins
To easily add and remove plugins you can use AutoPlugin2. the app also gives you a brief explanation of its purpose
Some useful plugins
By default when you mod the system you should already have several plugins installed such as henkaku, storagemgr, 0syscal6, nonpdrm, and a few others. the majority of the plugins listed in the install list are optional depending on your requirements and preferences.
DolcePolce
This plugin is only for the PSTV and is used to unblock games so that they can be run on the PSTV
udcd_uvc
Used to enable streaming of your Vitas display (no audio) to your PC over USB, basically your Vita will act as a live camera that can be used with tools such as OBS