interfacing means: Both interfacing and interlining, although invisible and unnoticed, are essential components in tailoring and dressmaking. The extra fabric layer between the top and bottom of garments — such as at the collar, cuff, or pocket — is interfacing. It provides additional strength and stiffness. The layer that is between the outer and top fabric of a garment, interlining gives it shape or strength. (in Fashion Dictionary)

What else does interfacing mean?

In order to stiffen, thicken, or strengthen a garment’s layers, a piece of fabric is usually inserted between them. (in Merlin Dictionary)