What is the big square on the free throw line do?

Answer 1

If you're referring to the "lane" or the "paint" it is the rectangle formed from the free-throw line, and perpendicular lines that extend from the ends of the free throw line to the endline. The official rule is that an offensive player cannot have any part of his body inside this "lane" for more than three seconds, otherwise a three-second violation is called. Also, when an offensive player is shooting a free-throw, no players can enter the lane until the ball hits the rim (High School rules).