Bifocals are lenses with two prescriptions on them. There is a line on the lens dividing the two zones, into distance (top) and reading (bottom). These lenses are usually prescribed for people over the age of 40, who need a little help seeing nearby objects.
Bifocals are easier to use than multifocals (which we will write about next time), as the different zones are very clearly demarcated. If you already wear spectacles to see far, and because of age you are now struggling to see near, bifocals are an option for you. The disadvantages of the bifocal include the limited distances you can view clearly (which is a problem for people who use computers) and that there is a clear and obvious line on the lens, which affects the cosmesis of the specs.