A needle is a small, steel tool almost as thin as a piece of cotton or string, and a haystack is a massive pile of dried grass in the middle of a field.
The chance of finding a needle lost in a haystack is very, very small. Almost none at all. And this is what the idiom ‘It’s like looking for a needle in a haystack’ means. It means you are looking for something you have little chance of finding. First listen to Jenny and her friend Zak, then listen to Peggy and her mum.