A Flexible Spending Account, also known as flexible spending arrangement is a pre-tax savings account used to pay for medical and health expenses.
It is money you ask to be deducted at the beginning of every Plan Year (you decide how much) from your paycheck and deposited into your FSA for the next 12 months to cover medical costs.
That money you decide to put in a FSA is taken from your pay before your social security, state or federal taxes are deducted.