It is the average chord of the wing. I you had a straight accross leading edge and a straight accross trailing edge then the wing would have no sweep or taper. In that case the MAC would be the same as the root and or tip chords and you could figure your CG as a percentage of the root or tip chord.

If your wing is swept or tapered or both then you have to find the average or mean aerodynamic chord and figure your CG as a percentage of that. It sounds more complex than it really is. It is very simple.

Ted