The result of the RAND function.

The result of the RAND function is not being displayed, but is being used inside the logical test of the IF function.

The RAND() function returns a number >=0 and <1. It stands to reason then that it would return something less than .5 about half the time.
There is no "50% of" anything. There is a random number between 0 and 1. If this time it would be less than .5, your formula returns "yes", otherwise, it returns "no".