Is it fair to say that if you don't know what it is, you probably don't need it?
Dimensions, generally speaking, are areas of your business you'd like to track in order to separate their cost or profit contributions to the company.
Cost Centers, for example, may be departments like R&D (Research and Development), Marketing or Customer Service. These are areas of your business which do not contribute directly to profit but have a cost to the business. This is in contrast to Investment and Profit Centers, which could also be set up as dimensions, but contribute to a company's profits.
You may, for example, want to have Sales and Service separated. If they share many of the same items and GL accounts, set them up as separate dimensions and you can better reconcile their financial contributions.
A division of the company, or any indirect revenue stream, could be set up as a dimension, too.