The Cm as I understand it, is the "turning moment" around the C/4 point that the airfoil develops at given angles of attack.

If you have a NACA0008 section at 0* then the moment (tendancy for the wing to rotate downward) will be zero. If you increase the angle to 2* then there will be lift generated but the airfoil will try to twist downward.

The effect of this on the overall scheme of things is, as I understand it, quite small for most airfoils in the model world.

My own experience is with ff glider using high camber (5-7%) airfoils. Part of the set-up in those particular models is the down moment of the airfoil which is comparatively large and is one of the factors in allowing the CG to be well back (like 65%-85% C is not uncommon). BUT that is a very specialised use and not one to aim for with other aircraft.