The wind angle at which the ship gets the most speed. The wind direction starts at 0 degrees. The best point is a certain number of degrees clockwise or counterclockwise from 0.
To put another way, it is the number of degrees (left or right) from your ship's bow the wind must be coming from to achieve your best speed.
Ships with predominately triangular rigged sails usually have a best point of 90 degrees - the wind should be blowing on the side of the ship to achieve best speed.
Ships with predominately square rigged sails have a best point of 135 degrees - the wind should be 45-degrees off of the stern of the ship to achieve best speed.
Also see the Sailing Guide.