Mean aerodynamic chord (MAC) is a techie term for "average chord". Finding the MAC for a straight, constant chord wing is pretty simple; it's just the chord section half way out the wing from the centerline of the fuselage (we have to pretend that the wing goes all the way through). But for a swept wing, non-constant chord it is a little more difficult and requires some geometry and trigonometry we have long forgotten. However, here is a relatively simple graphic method that requires only a straight rule, a sharp pencil, and some graph paper and not much brain power.
OK, why do we care? Well, if your plane is new and came packaged in a nice box with nice plans and an instruction book we don't care. But if your plans are not in English or you bought your new pride and joy from someone who threw out the plans you DO care. (long pause while audience waits breathlessly) .....because before you throw it into the air you MUST find a good balance point (C.G.) to start with. And you do that by finding the mean aerodynamic chord and using that as a basis.
If you want to be "safe" test fly your baby with the C.G. about 25% aft of the leading edge of the MAC. For a flying wing start out about 16% to 20% aft of the leading edge of the MAC. These measurments will be a little nose heavy, but better that than tail heavy. Move the C.G. back 1% at a time until you reach a point where the trim charts (see "Trim for Flight" section) indicate and you feel comfortable. Remember the farther back you move the C.G. the more elevator-sensitive it will be.
Now get out your graph paper, find your neutral point, draw the MAC through the neutral point parallel to the fuselage centerline, measure your percentage back from the leading edge of the MAC, and project that point over near the fuselage on both sides. Now you have a couple nice places to put your fingers when you check the balance.