The minor IV chord is normally a IV chord in a minor key but is often used in a major key for dramatic movement:
I – IV – IVm – I or simply I – IVm – I
When used in a major context it is said to be borrowed from the parallel minor key. If in A major the parallel key would be A minor and the parallel IV would be D minor. The use of this chord creates chromatic movement between the VI and V degrees of the major scale.