Essentially, a modifier occurs within a phrase containing the other form to which it relates. Thus in
The red ball is on the table.
the adjective 'red' modifies the noun 'ball', both being parts of the underlined noun phrase 'the red ball', while in
The ball is red.
the same adjective serves simply as a complement. It still relates to the noun, but no longer modifies it.
I hope that gives you a good basic idea!