The term trifocal is used to describe a lens in which the top portion of the lens has the prescription for distance vision, the middle portion of the lens has the prescription for intermediate vision, and the bottom portion of the lens has the prescription for reading. The three areas of vision are divided by visible lines in the lens. Trifocals let you focus through three different prescriptions at different distances through the same lens — hence the name trifocals.
The difference between lined bifocals and lined trifocals is the added segment in the trifocal lens. The added segment in the lens provides clear vision at arm's-length distance (commonly referred to as intermediate distance), the area that is usually blurred for lined bifocal wearers.