The polar diagram's axes are Cl and Cd. Where
the line crosses the Cl=0 axis there is no lift, and
therefore no induced drag, so Cd at that point is from parasite drag only.
Angle of glide is Cl/Cd, the lift/drag ratio. If the
lift/drag ratio is 20:1 then the glide angle is 1 in 20. The tangent to the
curve from the Cl/Cd origin shows the best Cl/Cd ratio, and thus the best glide angle you can get.
Rate of descent would depend on what speed you were doing down the hill and
that is not shown on the polar diagram.
The polar diagram shows coefficients only. To get real world data you
have to add in all the other bits of the lift and drag equations like S and rho
and V2.