Rake describes the slant of prop blades in relation to the hub. Rake affects how water flows through the propeller and can impact the boat performance.
Aft Rake helps to trim the bow of the boat upwards, which often results in less wetted surface area and therefore higher top end speed. Aft rake propellers also typically "bite" better on ventilating type applications.
Forward, or negative rake, helps hold the bow of the boat down. This is seen most often on workboat type applications.