"Link" is short for "hyperlink". Hyperlinks allow you to move between webpages by clicking on them. At Wikiversity, hyperlinks look like this: Hyperlink and Hyperlink or sometimes this. Most links are blue and will take you to another page if you click on them. Red Link City has information about red links.
Wikiversity allows subpages. You can link to subpages using Link/Subpage test or just /Subpage test.
Hyperlink - article at Wikipedia.