A WordPress Plugin is a program, or a set of one or more functions, that adds a specific set of features or services to a WordPress site.
There are over 30,000 free plugins available (see the WordPress plugins directory). Generally, you don't need to download these free plugins because you can search for and install them directly from within your WordPress admin area. There are also many paid-for plugins available.
For more about plugins, read WordPress Plugins: adding new functionality to your site.