Hey there! Okay, so an index catalogue is basically like a big list of stuff. It helps people find things more easily because they don't have to search through everything one by one.
Let's say you have a lot of books, but you want to find a specific book about animals. A normal person might have to read the title of every book until they find the one they want. But with an index catalogue, it's like having a cheat sheet! You can go to the index catalogue, look up "animals," and it will give you a list of all the books about animals. This way you can find what you're looking for much faster.
An index catalogue can be used for all sorts of things, like finding a specific word in a book or looking up a type of product in a store. It's just a really helpful way to organize stuff and make it easier to find what you need.