What is mean by the term fan out in microprocessor?

The term Fan Out is a measure of how many loads a pin can drive. This is usually normalized to the load of a standard TTL input, which is considered to be a Fan In of 1. This is a digital logic term, not necessarily just a microprocessor term.

Fan In and Fan Out are important, because you do not want to exceed the rated load placed on a pin without providing extra buffering.