Use this expression to talk about something that you were planning to do next. For example, at work:

A: Hey, are you busy?

B: Actually, I was just about to leave. What do you need?

This expression is useful in situations where you're a little bit late doing something:

A: Hey, can you send me those sales projections?

B: Yes, sorry. I was just about to do that.

This makes it seem like you were already planning to send them soon, even if this person didn't remind you.