The idea behind что is just 'what' as in 'what did you say?' - что ты сказал?
In a question it enquires after something in general.

another meaning is "that" as in "I told you that ..." - я сказал тебе, что ...

It's like asking 'what type ist it?' or 'which one is it?' instead of 'what is it?'.
Which one is it? - it's for КОТОРЫЙ
Какой usually asks about a quality or a property, or when you can choose out of several variants.