A plugin is software that you can “plug-in” to a self-hosted WordPress website to add new functionality or extend the capabilities of built-in features.
Plugins are developed by the WordPress community, as well as professional software and web developers. Because of how large the community is, as well as how long it’s been going, WordPress has tens of thousands of plugins. If you can think of a feature that you want for your WordPress website that isn’t part of WordPress’ feature set, it’s highly likely that there’s already a plugin that does just that.
The vast majority of WordPress plugins are free, but there are still thousands that you have to pay for. While plenty of free plugins provide value, you can’t expect all of them to be quality or have the exact features you’re looking for. Premium plugins are typically worth the upfront cost for their reliability, expanded features, and technical support lacking from free plugins.
Most plugins work seamlessly with WordPress when you install them, so you rarely have to do anything more to get them to function properly. However, with the sheer amount of plugins available, there is bound to be overlap between some of them regarding their functionalities and how they integrate with WordPress. This can lead to compatibility issues, which can be sorted out with the help of tech support, or avoided entirely if you carefully pick out which ones to use.