First and foremost I would think it would be wise if I explain what WordPress is, just in case you do not know. No biggie, WordPress is simply an open source framework for websites. Open source simply means it is free and has no fees for its licensee of use. This is great because now people all over the world are encouraged to develop for it. Because so many people do use WordPress worldwide we have an abundance of amazing themes and plug-ins to help sculpt the perfect website.
Plug-ins are small modules of code that are easily integrated into a website that add various functionality or styles. An example could be a contact form, or a special type of photo gallery. More complex and specialized plug-ins can add a certain style of button or a perhaps a reservation booking system. The one thing you need to understand is that literally any need you may have for a website, there are dozens of plug-ins available to achieve that function or style. This is the true beauty of WordPress.