The number comes from multiplying your Adjusted Gross Income on your Form 1040 Line 11 by 7.5%, If your total medical expenses are not greater then that amount then there is no need for you to enter you medical expenses.
Health care insurance premiums and other medical expenses that you paid with out of pocket funds are an eligible medical expense that you can deduct using Schedule A for itemized deductions. However, only your total medical expenses that are greater than 7.5% of your Adjusted Gross Income (AGI) can be deducted.
