Definition of raise
A "raise" means a raise to poker. A "raise" means adding to what has already been put. We can raise at any time, except when our carpet does not allow it to us (when we are all in). If you are playing Hold’em without limit on a €1/2 table for example and you want to make a raise preflop, your minimum raise will be 2 times the big blind or €4 and your maximum raise will be equal to the size of your stack.
Reasons to make a raise
This short paragraph will not cover all the reasons to make a raise, but let's name a few to get an idea. A raise can be used to reduce the number of players in the hand. For example, if 4 people have already limped and the floor is yours, you could roll a raise to get 1 or more of those players to fold. You will then have a better chance of winning the hand by decreasing the number of players in the hand. You can also do a raise to flop because you have a good hand. You can also roll a raise on the river, for example, in order to bluff your opponent.