[damnitpeter]

Mode is simply the wavepattern of the light traveling through the optical fiber. Multimode means there are a few different wavepatterns traveling down the fiber, they tend to potentially interfere and its not as efficient of a use of optical power. In singlemode, this is a lot harder to do and requires better optics, the light is in one pattern, power is efficiently allocated to that pattern, and thus the light can be sent much further. Its been many years since college so hopefully that explanation suffices.

Now lets say you want to send multiple signals to get more bandwidth on one fiber, you just need to move the frequency/wavelength of light so different signals have different wavelengths and can be discerned at the receiving end. That's gonna take even more optics and filters on both ends, but works quite well to add more bandwidth to existing fiber installations where running more fiber would cost a lot more than installing new equipment on the ends.

[Hikikomori]

Rather than interfering with each other multimode suffers mainly from modal dispersion as the modes do not travel the same path and some take a shorter or longer one, so the signal is spread out.