The Sicilian Defense is one of the more common openings for black against white playing e4. To counter this white usually tries to take away the initiative from black. The Rossolimo Attack does just that back bringing the bishop to b5 if black plays Nc6.
There are a few other setups where you can play the Rossolimo but I really only recommend it if your opponent puts his knight on c6. Regardless of where your opponent plays next, white looks to take the knight and double up the pawns on the c-file. After black recaptures white will solidify his pawn chain with d3 and continue his development.
It is difficult for Sicilian players to maintain the pressure they are used to against the Rossolimo Attack. It should be in every arsenal as players playing as white.
Watch the video below to learn more about extended lines in this opening.