Difference between OM3/4 is bandwidth and distance. Both are multimode fibers. Multimode is mainly used between closets, to workstations, etc. Most cases in AV work, you will use OM3 multimode.

Singlemode is used for very long distances. We install this in between buildings that are on the same street or campus. Can go even further like miles.

The easiest explanation between multi and single is this:
Imagine a highway tunnel through the city (the actual glass fiber itself). In multimode, you will see your normal traffic of cars all over the place heading in the same direction. In single mode, it’s just your car and no one else’s.
This is the reasoning of why tx and rx strands are required for full duplex communications.

Hope it helps. Fiber is the future and uses less resources than our normal copper based products.