We’re talking about the colon that’s used as a punctuation mark!
The grammar doctor encourages you to use colons so you can practice varying your sentence structure, but you need to use them correctly. So here’s how to use them: The number one rule is that you must precede the colon with a complete sentence. Then, just be sure that the text after the colon either defines or clarifies the text that came before it.
Last summer, I visited three states: Louisiana, California, and Texas.
In this example, notice that we have a complete sentence before the colon (Last summer, I visited three states). Then, the text following the colon defines or clarifies the complete sentence.
Last summer, I visited three states: [What were those three states?] Louisiana, California, and Texas.
Try out this usage if you want to take your writing to the next level!