Are colons used to add an explanation of secondary importance to the sentence?
A colon is used to separate two independent clauses of which the second explains, illustrates, or expands the first.
A colon is a punctuation mark that connects two independent clauses.
The second clause is not necessarily of secondary importance. It is important because it adds to the first clause by way of explanation, illustration, or expansion.
Often the second clause is modified into a phrase since it leads from the first clause.
1. The team knew what was essential: daily practice for at least three hours.
2. For the trek, carry only these items: a rainproof jacket, energy bars, water, and a camera.
3. She received the award for best research paper: she had worked hard all semester to accomplish it.