Borrowing from sports (Cricket, actually), spin is the rotation a bowler gives to the ball at the time of delivery.
It means the ball does a complete 360 degree rotation, or multiple 360 degree rotations per minute (as high as 2,300 rotations per minute) depending on the skill and experience of the bowler. This is called spin.
Another example would be the Earth’s rotation. The Earth spins on its axis.
When the ball is thrown, normally, it moves in a straight path. But sometimes, the ball, which is travelling in one direction, after bouncing, changes direction. This deviation of path from the original path, is called turn.