A plugin is extra program code that allows an extra feature or some extra functionality. Think of it like an extension to a room or house. Some examples include: forms, booking, newsletter, SEO and spam shielding. WordPress has many wonderful features built in, but plugins add the icing-on-the-cake and freedom to customise. Plug-ins can be free or low cost.
But there are dangers not the least being just a waste of your time. The other concern is code that may introduce a security hole. These holes are usually solved by updates. Therefore choose a plugin that has a good rating, been around a while, and is updated regularly – indicators of a good one. There are thousands to choose from. If it is too confusing or bewildering, let us check or suggest plugins for you.
Search the official directory of WordPress plugins for a great start.