Definition of reraise
A reraise is when a player raises. This raise is also called a 3bet. Let's say on the flop a player bets 5???. This bet will be the first bet. Then, let's say a player raises to 15???, that would be the 2nd bet. And if a player over-raises (re-raises) to 40??? for example, this is called a 3bet or re-raise. As a general rule, if you are playing small limits, this is a play that shows great strength. When a player reraises, it is, most of the time, that he is ready to go all-in.
Reasons to make a reraise
If you are at the micro limits or small limits, generally you should reraise because you have a good hand that you want to be all in with. So you do it in value. You will do it as a bluff against some good players with whom you have a dynamic. You can also reraise to protect your hand. Let's say you feel your hand is the best on the flop, but several cards on the turn could kill your action, you will usually favor a reraise over calling your opponent's raise.