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Check out this video on better jumps for platformers - 

But basically coyote time is when your player leaves a surface and hasn't jumped. You still allow them to jump for a mili-seconds in the air. This helps with slow reaction times and fps problems. 

=> OnCollisionExit with floor -> IEnumerator - yield for seconds (0.2) -> If jumping is == false then set jumping to true.

(within those 0.2 seconds of airtime, if player hasn't jumped, then still give them the chance to jump)