During a soccer game. When one team is given a free kick, the opposing team sets up a "wall" of 3 to 6 players 10 yards away to prevent the player taking the free kick from kicking the ball directly at the goal (assuming the free kick is close enough to the opposing goal). Great players, like Beckham, when taking the free kick will bend or curve the trajectory of the soccer ball around the wall of oppossing players and often into the goal. He does it better than most players to the point wheRe it seems impossible.

In Germany they market it as "Kick it like Beckham" throwing out the metaphor.

As has already been mentioned, the
literal
meaning of the film's title is David Beckham's ability as a free kick specialist to "bend" the ball around defenders and into the goal.
However, the title is also obviously meant as a metaphor. Gurinder Chadha has explained in interviews that it refers to the relationship girls like Jess have with their cultural identity, and how they don't have to break away from it, but can "bend around" the rules to achieve their dreams.
'Well Bend It Like Beckham refers to Beckham, David Beckham, who is the Michael Jordan of soccer in England. He has this great skill in that when he kicks a ball, instead of going in a straight line, it can twist and bend into the goal, so hes famous for bending the ball, which I think is a great metaphor for a lot of us, especially girls. We can see our goal but instead of going straight there, we too have to twist and bend the rules sometimes to get what we want.'
http://blackfilm.com/20030314/features/gurinderchadha.shtml

If the game were golf the kick would be called a 'hook'.

Bend the ball. It's like a straight kick that really has to turn (bend) to make it to where you want it to go - usually the goal.