It's really simple, the triangle represents a theory related to computers and how Time, Budget and Specification relate in the development phase. Here are some examples:

• If you are working on a project, and have little time to implement it, it is important to have a large budget and a great set of specifications.
• If budget is a concern or smaller than you would like, be prepared for a long time for implementation even with a great set of specifications.
• If you have bad specifications (you don't know what you want) it is going to be expensive and take a long time to do it.