Indicates a person's intention to leave shortly for another destination, often casually. Just to let you know, I'm off for a meeting in ten minutes.
Pro Tip 2/3
Immediate Future
Often implies an action that will happen in the near future, not necessarily immediately. We're off for our vacation early tomorrow morning!
Pro Tip 3/3
Casual Tone
Used in informal situations, like talking to friends or family, rather than formal occasions. I'm off for a quick lunch break, I'll catch up with you later.