The colon is used following a complete sentence and introduces a list, a definition, a quotation, and, sometimes, a contradiction. It is an abrupt stop that signals that something is to follow.

For this class, you will need the following materials: pens, pencils, erasers, chalk, and charcoal. (List)

The meeting had only one purpose: agreement upon a suitable location for a new baseball stadium. (Explanation)

Although nobody knows who Murphy was, his law seems to be a truth: If anything can go wrong, it will. (Definition)