The Open Sicilian is a large branch of the Sicilian defense.
1. e4 c5
2.Nf3, the move that signals an open sicilian.
Black has many different replies to this move.
The main replies include Nc6, d6, and e6. These variation possibly transpose, but also can lead to different formations, like d6 leads the the Dragon and Najdorf, etc, e6 leads to the Kan and Paulsen, etc, and Nc6 leads to the Lasker-Pelikan and Kalashnikov, etc.
The main purpose of Nf3 is to do d4, which opens up the center. It takes over the center and gives white a good d4 knight, but he lost a center pawn for a less valuable c pawn. It is an interesting system which is very complex.
Thanks for reading, enjoy and have fun!