Why Multipath means copies of the same signal arrive by different routes, so they show up at slightly different times and phases. The delayed copies smear into the following data symbols and can partially cancel the direct signal, so the receiver misreads bits and the error rate climbs. This is why packet and digital modes often need retries or error correction in areas with lots of reflectors like buildings and terrain.
Watch out The idea that FM is immune is wrong: FM's capture effect helps with weak interfering stations, but delayed reflections of your own signal still distort the data. And multipath does not force any particular change in baud rate by the number of paths; higher rates are actually more vulnerable because symbols are shorter.
More paths, more smeared bits: multipath means more errors, not a new speed setting.