It's a way of expressing the future: He's about to retire. = He might retire soon. = He plans to retire in November. = He's going to retire soon. So 'be about to do something' means 'to be close to do something; to be going to do something very soon' (used for an action in the near future). We can use 'be just about to / going to' for the very near future. The coach is just about to leave / just going to leave.

be about to = be going to

It means what's up -What are you going to..... _Ready to do something

"you are about to..." means that something is 'almost' happening to you. For example: You have made a lot of mistakes, you are about to lose your job (You've made a lot of mistakes, you're almost losing your job)