As the eyes age, a stronger prescription is often needed to read. The stronger prescription that allows for reading makes it difficult to focus on objects at intermediate distances, such as on the computer or your speedometer. Thus, trifocals were made to offer a third prescription for intermediate focusing.
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 Bifocals and Trifocals is dealing with the lines between the different focusing powers. Fortunately, recent advances in technology have led to the development of the progressive lens.