In AutoCad the syntax for Relative Polar Coordinates is (@ distance < angle).
So in command 1,8 to @5 <10, (1,8) will be relative polar coordinate where x=1 and y=8 and in (@5 <10), 5 is the distance at an angle of 10 degrees from the relative coordinate. Therefore the length of the line would be 5 (Five )units.