What is a 1040 x tax form for?

Form 1040X is used to amend (correct) a previously filed return. For example, let's say you filed your return as soon as you got your Form W-2 from your employer. Then later you get a Form W-2 from a temporary job you had earlier in the year that you forgot about. Now you must amend your return to include the income from both jobs. You'll use Form 1040X to make that correction and add the income that was missing on your first return.