JEE basics: what is the difference between displacement and distance for a body thrown up and back?

Distance is the total length of the actual path travelled, so it only adds up and can never decrease. Displacement is the straight-line vector from the starting point to the final point, with direction, and it only cares about where you start and end, not the path. When a ball is thrown straight up, say to a height h, and returns to the launch point, the path it covered is h going up plus h coming down, so the distance is 2h. But the final position is exactly the starting position, so the displacement is zero. This is why distance is a scalar that keeps growing while displacement is a vector that can be zero even after a long journey. The same idea applies to round trips in any direction.