Many article titles are disambiguated with parenthetic qualifiers, like this: [[Self (psychology)]]. But when you want to include such a link in the body of an article, this would look rather awkward. So all you have to do is use the "pipe trick", like this: [[Self (psychology)|]]. Notice the "|" character stuck in there at the end of the link? That makes the link look like this: Self, without having to type the name of the link after the pipe!
This trick also works with namespaces, so that [[Wikipedia:Tip of the day|]] (again notice the pipe character) displays like this: Tip of the day.
This is the tip of the day (a.k.a. TOTD) project, providing useful daily advice on how to use or develop Wikipedia more effectively. This project is responsible for maintaining the Wikipedia:Tip of the day#Tip templates, and the collection of daily tips that are displayed by those templates.