arc4random returns an integer between 0 and (2^32)-1 while arc4random_uniform returns an integer between 0 and the upper bound you pass it.
From man 3 arc4random:
arc4random_uniform() will return a uniformly distributed random number less than upper_bound. arc4random_uniform() is recommended over constructions like ``arc4random() % upper_bound'' as it avoids "modulo bias" when the upper bound is not a power of two.
For example if you want an integer between 0 and 4 you could use
arc4random() % 5
or
arc4random_uniform(5)
Using the modulus operator in this case introduces modulo bias, so it's better to use arc4random_uniform.
To understand modulo bias assume that arc4random had a much smaller range. Instead of 0 to (2^32) -1, it was 0 to (2^4) -1. If you perform % 5 on each number in that range you will get 0 four times, and 1, 2, 3 and 4 three times each making 0 more likely to occur. This difference becomes less significant when the range is much larger, but it's still better to avoid using modulus.
