If you start at (0,0) and increase 2 units in the x-direction on both lines, Line B goes up 2 units in the y-direction, while Line A only goes up 1 unit in the y-direction.
Put another way, if you started at the origin and were walking up either line (pretend their two hills), your elevation on Line B increases more quickly because it's a steeper hill.
So Line B has a great rate of change. The rise compared to the run is greater.