What is an index for?

I think you probably asking about index of computer or software. Index is used to manage the files by putting them in a certain order.

For example, in a laptop, there are so many files and data. If it does not use indexing to put all the files and data in a certain rule, it will be take a long time to find a certain data we are looking for. So the laptop operating system will "indexing" all the data to make the searching much faster.