If you are about to do something, you are ready to do it/ close to doing it… you are on the verge of doing it… you will do it in the next instant.
(Estar prestes a fazer/ estar para fazer/ perto de realizar)
1. Sometimes when you are just about to call someone, they call you first. We can say that they “beat you to it” (They were quicker).
2. When someone mentions something that you have already been wanting to say, it’s common to respond, “I was about to say the same thing!” It is commonplace in conversation.
3. My passport is about to expire, which means I will have to take a trip soon to Brasilia, to the South African Embassy soon. Do you have any contract, membership, document etc. that is about to expire?