Selenium is a project created for automating web browsers, although nowadays it's used for automating more than just web browsers. You can use automation for most things on the web (such as automating administrative tasks that involve lots of digital paperwork, or doing something silly), but the top usage for automation tools is to run tests on the web.
To start with automation, I'd recommend learning about the Selenium WebDriver APIs first, since Appium implements most of the WebDriver API. For this, all you need are a modern browser (preferably Firefox 47.0.1) and the JDK set up. Optional, but recommended, tools to have are a good IDE (use your favorite if you have one), and either Gradle or Maven. You should create a project that will launch create a Firefox session, navigate to your favorite site, and perform some arbitrary click and type actions. This will give you a good stepping stone to knowing the APIs. I wouldn't spend more than 1-2 days on this step.
