Once upon a time, a long time ago, the military received their pay once a month. I've heard stories of lining up at the pay office to receive their cash..$18, $24, $53. Things sure have changed. One big change to military pay happened when service members were given the option to receive half their pay 15 or so days early - a mid-month paycheck. Though most people now think this is usual and the way things are regularly done, historically it is an optional benefit to help military families who might have difficulty budgeting across an entire month.
When you receive mid-month pay, what you are receiving is an interest-free advance of half the estimated amount of total pay that you will receive for the entire month. It is pretty clear on the LES - the mid-month pay is listed as a deduction from the month's total pay. 
Here's the interesting thing, though...you don't HAVE to get paid twice a month. If it works better for your family, you can choose to do the once a month pay option, if you are in the Army or the Air Force. Some people think it is brilliant, some people think it is useless. I think it is nice to have choices. (I have no idea why it isn't available to those in the Navy or the Marine Corps.)
If you think you would like to switch to getting your pay only once a month, remember that it may feel like you've missed a paycheck when you get started. Depending on when the change goes through, you'll likely "miss" either this month's mid-month pay or next month's mid-month pay, with the entire pay amount arriving on the first of the following month. This is not going to be easy if you are living paycheck-to-paycheck.
The form used by the Army is DA Form 3685. I don't know the Air Force equivalent form. Your finance folks should be able to help you make the switch, and if they can't, ask them to find out how. It can't be done via MyPay or any other way than through finance.
