Interfacing is applied to the wrong side of the fabric, to give structure and/or strength.  It is often used on waistbands, neck facing, button hole areas etc.  As a rule of thumb, use interfacing the same or lighter weight than the fabric you intend to apply it to.  Fusible interfacing has a heat activated adhesive on the back, which when placed against your fabric and ironed cause it to adhere.  Always use a cloth over your interfacing, you will be very glad you did, if you find you placed your fusible interfacing the wrong way round!