Guard, round, sticky bits (floating point)

I don't understand what is the meaning of guard, round and sticky bits. I have an example: if we add 3.25+30 (3 bit mantissa) we get 32 because grs=011. I was trying to solve it but all I have managed to do is this: 30=1.111*2^4 3.25=0.001101*2^4 and if I add up the numbers the result is 1.0000101*2^5, which means g=0 r=1 s=1. The problem is now that I can't figure out why is the correct answer 32.