Hello Brian,
CG is short for Center of Gravity. It is probably the most important thing to make sure you have right in order for any plane to function. The center of gravity is usually 30% back from the leading edge of the wing at the fuselage. I say usually, because 30% is a good starting point. However, it may take some adjusting which comes with experience. You learn to get a feel for how planes act when either nose or tail heavy and correct when needed. Your cubs manual should list the recommended CG. Along with control throws etc...

~Noah

***EDIT***
To Clarify: 30% of the chord (from leading edge to trailing edge) of the wing at its root (or where it meets the fuse)