Noch means something like more, but in an adverbial way, as in the expression children hear from their Omas: “Iss noch!“–“Eat something else!” Wieder means again, as in the expression everybody knows even if they don’t know its exact meaning: “Auf Wiedersehen!” — “Until the next meeting, until we see each other again!” And immer means a whole lot of adverbial things like ever or always. Now let’s put them together.
If you continue in the same state, you use immer noch, and it means things are still like they were before. I’m reminded of a reproachful comment from a neighbor who walked by my house at midday and found the shutters down: “Immer noch zu!” — “Still closed!” (This implied that I was still sleeping, or at least a careless housewife.)
But what if the state you’re continuing isn’t constant? What if it’s something that repeats, so that there’s action–rest–action–rest? Then you use immer wieder, and it means that you continue to repeat something–you do it again and again. “So hab’ ich immer wieder gesagt,” grumbled an old man to his wife as he walked past me yesterday. “That’s what I’ve said again and again!”