A colon (:) is a tip-off to get ready for what’s next: a list, a long quotation, or an explanation. It’s used to separate independent clauses when the second clause explains or amplifies the first.

• “Fred was proud of his sister: she had been promoted to managing partner.”
• “My new office contains the following items: a partner’s desk, a leather chair, and oak paneling.”
• “We need additional information: escrow statements, tax returns, approved bank loans, and mortgage agreements.”
• “There are two things to remember in a job interview: always arrive promptly, and always dress appropriately.”

Other common uses include:

• After the formal salutation in a business letter
• Before a list
• To separate hours and minutes (depends on culture)