The amount that a line "rises" between two points on a line. This is, specifically, the change in the y value between the two points (delta y).
If our line includes points (2,4) and (12,17), our rise is 17-4=13. 4-17=(-13) also works, but it usually makes more sense to subtract the smaller number from the larger (to get a positive value). Note: if you calculate rise by doing y1-y2, don't calculate run by doing x2-x1.
Rise is used in the calculation of slope.