Something that is dominant is the most important. An art work has dominance when one part of it is more important than any other part. Various aspects can be dominant:
Designs are often more interesting to look at when they have dominance.
Again, what is dominant is a matter of judgment and feeling.
Go on with Design and Pattern Study
The link for the Line Dominance example is to a drawing which my younger son made at the age of 4 using MacPaint about six years earlier. I couldn't find any works in various museums on the web which were really good examples of line dominance. Fortunately, I had our "electronic family scrapbook" at my disposal.