Usage of 的要 in 頭髮太長囉，該剪的要剪一剪了
"的要" is not a term; "的" makes the phrase a noun, so "該(should)剪(cut)的" means "what should be cut". "要" means should/must/shall/will, so the whole sentence literally means "(you) should cut (the long hair) what should be cut".