Bifocal is a compound word, "bi" meaning two, and focal referring to the focal length. Bifocal glasses have two focal lengths. One focal length allows the wearer to focus at distant objects. The other focal length allows the wearer to focus on closer objects, such as a newspaper or computer screen.
When we're younger a normal eye has the ability to change it's focal length. When everything is "young" and working properly, we can alter the shape of the clear lense inside our eye thereby changing the focal length whenever we want to look at something near or far. As we get older, we loose that ability. Somewhere around 45, most of us start to have trouble focusing on closer objects. That's because the lense INSIDE our eye can no longer change shape on demand the way it did when we were younger.
Bifocal glasses are a way for your eye doctor to provide you with a tool to help you see at a distance and up close.