Plugins contain code that adds extra features to your WordPress website.
There are thousands of plugins available from many different developers, but you do have to be a little careful as not all of them are well written and the code could even contain malware.
A good, and relatively safe, place to look for plugins is the WordPress Plugin Directory, which includes reviews by other users. It’s always a good idea to check the reviews, to look at when the plugin was last updated and to see whether there are any outstanding issues on the support page.