"Left backs" refers to the players in the sport of football (soccer) who primarily occupy the defensive position on the left side of the field, close to their team's goal.
The versatile defender has been put at left back in the world XI but could just as easily have popped up in another position and it not be a surprise. (caughtoffside.com)
The dude is so freaking consistent, he has to be the best left back in the country. (justarsenal.com)
Right and left back positions are as competitive as I can remember us having. (justarsenal.com)