Monsieur Martin,

It's all based on Logical operation.
1 AND (&) 0 =0
1 AND (&) 1 =1


3F = 11 1111

let's say MyRandomVar return 23121

23121 = 101 1010 0101 0001

3F = 000 0000 0011 1111

result = 000 0000 0001 0001 = 11 Hex = 17 Decimal