Competence enables native speaker to recognise ambiguous sentences or accept even apparently meaningless sentences as syntactically correct (and even making some sense).
She read the book I gave her last week. and
Colorless green ideas sleep furiously. 4
Performance may be flawed/ ungrammatical, due to attention/ memory lapses or other psychological factors - so cannot be taken at face value. The ``raw data'' has to be ``idealised''.
Like the earlier structuralists (Bloomfield and Harris) Chomsky focused on the formal structures of language and claimed the autonomy of syntax (from semantics and pragmatics of language). But unlike them, Chomsky challenged behaviourism and emphasised creative open-endedness of syntax.