I thought "get open" mean "to open something", but what does it mean here?
"Get open" is not meant in the literal sense in this context. It needs to be understood in the sports context. They are playing football here, and in football (and basketball) terms, to "get open" means to get away from defenders into an unguarded area on the field (or court). Similarly, "go long" is also another sports term which means to run far down the field away from the quarterback, to receive a pass. "Get open" is not meant to be understood in the general sense, which is to open something.