in easy words WordPress plugin is something that adds new functionality to your WordPress site or extends existing functionality on your site.

WordPress plugins are written in the PHP programming language and integrate seamlessly with WordPress. In the WordPress community, there is a saying that goes around: “there’s a plugin for that”. They make it easier for users to add features to their website without knowing a single line of code.

The official WordPress repository has more than 45,000 plugins you can choose from and use in your site ,but you may need to add customized functionality to your website, which may not be available in any existing plugin ,in this case you would need a specialized programer to create a new plugin didcated for your site .