Multifocal lenses (sometimes called Varifocals) were developed in the 60's as a type of blended bifocal to make them more like natural vision for people who needed a reading addition.

There is a gradual increase in the power of the reading addition as you go down the lens so the near focus get closer.
This means that whatever the distance, be it 5ft or 3ft, there is a part of the lens that brings it into clear focus.

Also the area of clear vision gets wider as the full reading addition is reached.
At the normal reading distance the area is wide enough for a normal line of print, although some head turn is necessary for a newspaper.

Unlike bifocals the multifocal lens does not cause any sharp changes in focus. There are no missed steps or kerbs that aren't there as can happen with bifocals. Although there may still be some out-of-focus areas they are not normally troublesome.

The earliest design of "Varilux" were just bifocals with the line smoothed out. As such they induced quite harsh distortions into the side of the lens. Some people found that there was a feeling like sea-sickness. It is this problem that modern designs try primarily to overcome.
Even the most budget modern design is immensely better than this whilst the best designs are often totally free of noticeable distortions.