Resolution is the number of dots per inch. Since each dot represents a point in the picture, the more dots you have, the more detail you have. Imagine a grid of 3x3 dots. It's difficult to paint much of a picture with just those 9 dots. But now imagine the same size grid with 5x5 dots or 100 x 100 dots. The more dots you add in the same size picture, the finer the detail becomes.