Bifocals allow the wearer to read through one area of the lens, and to focus on distant objects through another area of the lens. As the eyes age, though, a stronger prescription is often needed to read. This would be fine, but the stronger prescription that allows for reading makes it difficult to focus on objects at intermediate distances, such as grocery items on a shelf or your speedometer. Thus, trifocals are necessary for a third prescription for intermediate focusing.
Trifocals, also known as line trifocals, feature three areas of focusing power, each separated from the other by a distinct line. The three windows allow for focusing on distant objects, intermediately distanced objects, and for reading. The downside of trifocals is dealing with the lines between the different focusing powers. Fortunately, recent advances in technology have led to developments in no-line, or progressive lenses.