Although the concept can be confusing, resolution is just a measurement of how many pixels a scanner can sample in a given image.
Resolution is measured by a grid.
Think of a chessboard, with eight squares along each side. The resolution of that chessboard would be 8 x 8. That scanner samples a grid of 300 x 300 pixels for every square inch of the image, and sends a total of 90,000 readings per square inch back to the computer. Generally, higher resolution scanners cost more and produce better results. Unfortunately, things aren't that straightforward in the real world.