What is the wall in soccer?
On a free kick near goal, defenders line up shoulder-to-shoulder to block the direct path. The kicker tries to bend the ball over or around them — and yes, they all guard their groins.
Explain further
The one-liner covers the basics, so here is how the wall actually works. When a foul gives the attacking team a free kick within shooting range, the defenders are allowed to stand between the ball and their goal, but they have to retreat at least 10 yards (the referee enforces this, sometimes with a can of vanishing spray to mark the line). Three or four defenders pack together to cover the part of the net the kicker is most likely to aim for, while the goalkeeper guards the rest. The kicker then has to beat both: curl the ball over the wall and down under the bar, or bend it around the edge.
The wall exists because a free kick from 20 yards out is dangerous. Without bodies in the way, a clean strike would be nearly impossible for the keeper to reach in time. The wall shrinks the target and buys the goalkeeper a split second to react.
One thing that trips up newcomers: when the wall has three or more defenders, attacking players must stay at least a yard away from it, so they can no longer sneak in to disrupt it. And yes, the flinching and groin-guarding are real. A struck ball can travel close to 70 miles per hour, so bracing for impact is simply self-preservation.