To formally approve or agree to something. The manager needs to sign off on the budget before it can be implemented.
To end a broadcast or transmission, typically by a radio or television presenter. The news anchor will sign off at the end of the program.
To conclude a letter, email, or other written communication. She decided to sign off her email with a friendly note.
To stop work or an activity, especially at the end of the day. I usually sign off from work at 5 PM.