In a CMS ( Content Management System) a Plugin is a collection of code files that adds one or more features to your web site.
This is the basic meaning of “plugin.” It is basically software.
Why are plugins important?
Hubspot says Plugins are the building blocks of your WordPress site. They bring in important functions to your website, whether you need to add contact forms, improve SEO, increase site speed, create an online store, or offer email opt-ins. Whatever you need your website to do can be done with a plugin.There are thousands of free and premium WordPress plugins to download and add”
However, the meaning of plugin varies, depending on the CMS(Content Management System) you use. eg
In WordPress, a “plugin” is the general term for “code that adds a feature to your site.”
Visit the huge and growing WordPress Plugin Directory and browse thousands of free plugins.
But, be warned, some plugins when installed together will have a ‘conflict’ causing site issues.
That’s why Plugin selection and installation is best left to your web development team!