Definition of open raise
An open-raise occurs when no one has made a move before you. If you are in UTG position and raise, you are making an open-raise. If a player has limped in before you and you bet, it is not an open-raise, it is an iso-raise (you isolate yourself with the player who limped in). The term "open" indicates that you are the first to make a play. The open raise is a very classic poker play. Generally, when no one has spoken before you and you want to go to a flop, it is very advisable to make an open raise.
The advantages of open raise
An open raise has several advantages. First, you decrease the number of players who will see the flop. If you make a limp, it is possible that several players will call and you will see a flop with 4 or 5 players. The more players on the flop, the less control you have over the hand. Also, the more players on the flop, the less equity you will have in the hand. Second, when you make an open raise, you have the initiative, meaning you have control of the hand. You represent a strong hand and you can represent multiple strong hands on multiple flops.