The airlines will outsource your information to a professional background check contractor, unless they do it in-house (doubtful). They are checking that any and all information you have furnished is indeed true and correct. Any gaps in work history must be corolated by a reference to state that you were in training, school or on the job hunt. They will check your criminal history, if any, and if they have requested it, your driving record.
Most will be checked by telephone calls to your prior employers to verify employment dates and last position held at said company. The FBI database will be reviewed for criminal history. If you have been fingerprinted by the air carrier, they will verify those too.
No mysterious at all, just SOP, to ensure you are who you say you are and that you have not provided any misleading information that may disqualify you from being given the job you have applied for. It's half standard HR stuff and half security measures.
Key point: Don't fib on a job application. They will find it and you will be disqualified quickly and in most cases won't be given the specific reason. You'll just receive the old "we're sorry, but this position has received many applicants and the competition is at an all time high....you may re-apply in X months....we wish you luck in your career goals."