From the game manual:
Offensive Flow (how well the offense produces assists, avoids turnovers): point guard weighted very heavily, shooting guard weighted medium, small forward weighted medium.
Basically, your PG's handling and passing skills are the most important. Offensive flow is one of the ratings that I personally believe can be most deceptive because passing and handling is important on all players. Better to have a team full of 7 passing and 7 handling than a PG with 12 handling and 12 passing but everybody else has around inept and awful passing.