Pattern balance is a series of internal consistencies that help a garment made from the pattern hang correctly on the body, and not pitch forward or backwards or twist. Before I knew any formal patternmaking from scratch, I had figured out there were patterns that I could do all sorts of changes to, and they'd look fine, and patterns that you didn't dare change if you wanted it to stay on your body. Kwiksew were usually quite reliable; big 4 patterns much less so. (And I found out later that KS usually worked with patternmakers from RTW.)
Center front and back on perfect grain
Bust on perfect crossgrain
Shoulderblade level on back of pattern on perfect crossgrain
Side seams at the same angle(s) -- pin them together and CF and CB should be parallel. The front pattern piece is typically about 1/2" wider than the back. Pin the front armscye/side seam corner to the back armscye/side seam corner. Pivot the pieces until CF and CB are parallel. Take a look at the side seams -- they should match. If they don't, add and subtract to the side seam until they do (often the fast way to do this is to pin the two pieces together with CF and CB parallel, then cut the side seam front and back simultaneously with your scissors.
Pin the front armscye/side seam corner together with the back side seam/ armscye corner with the side seams "looking at each other", instead of on top of each other. The armscye should make a nice horseshoe shape, with a smooth U at the bottom. No V or A points, please. The length of the back armscye's seamline should be 1/2" more than the length of the front armscye's seamline.
The imbalances I typically see in commercial patterns are strange armscyes and unbalanced side seams.
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For pants, the balances are:
Perfect grain down CF and CB of the legging
Inseams of same shape and length and parallel to each other when you put front and back grainlines together.
Crotch seam not forming a A or V when sewn.
Side seams of same shape and length and parallel to each other when the front and back grainlines are together
Back hem length 1" more than front hem length.
More information: Connie Crawford's Patternmaking Made Easy.
Kay