A test plan is a tool you use while planning a period of testing. It basically describes what will be tested, who will do it, where they will do it, and how they plan on doing just that. In some coorperations, where law and contracts are really important, test plans are usually the size of a small child, and nobody ever reads it. Apart from lawyers. That's because test plans aren't always made for you - they can also be made for your team or for different stakeholders.
The short answer: A lot. In December 2015 I was called to the offices of a project, who went live with their website the month before. On the website, customers could search for and buy products that could be sent to other people via both physical and virtual shipping channels. And they wanted me to test the "user experience" of the website on different platforms and devices.