Usage of 的要 in 頭髮太長囉，該剪的要剪一剪了 I am having difficulty understanding how 的要 is being used in the second sentence:


  頭髮太長囉
  該剪的要剪一剪了


The first sentence makes sense: [Your] hair [is]  too long

However I don't understand at all how 的 and 要 are operating in the second one.  I was not able to find any information on it online (I may have searched for the wrong terms).
"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".