Agent May is an expert pilot and martial artist. Though she was once a legend in the agency, she mysteriously resigned herself to a quiet desk job in recent years.
Agent May has clearly endured some kind of trauma resulting in her withdrawing from field duty and isolating herself until Agent Coulson recruits her to “drive the bus” (fly the S.H.I.E.L.D. plane).
Coulson reassures Agent May by pointing out that the team he is putting together is just like they used to talk about, picking their own ops, making the calls and that there’s no red tape, so Coulson and May obviously have some past history together.