Spin Axis is the axis that the ball spins on or the measure of the tilt of that axis. A positive Spin Axis means that it is tilted to the right (a slice or face for RH golfers). A negative Spin Axis means that is it tilted to the left (a draw or hook).
It is often compared to the wings of an airplane. If the plane’s wings tilt to the left, the plane goes left. The Spin Axis works the same way.