"I am standing in line."
Translation:줄에 서 있어요.
It is used to express the state of a completed action.
Your action was to go stand in line, now you are in the state of standing there.
Maybe this example is a bit confusing, but it is the same as
문이 여려 있어요. The door is open (it got opened and now is in the state of being open).
More info here.