To reply to Oreneta, indexing is creating - in most cases - the index in the back of a book. It involves going through the text and making decisions about which terms need to appear in an index, which term to use in the index if more than one is available, do you use cross references or duplicate entries? You have to structure the index depending on the subject, whether there are illustrations or maps etc.
There are other sorts of indexes as well There are journals, conferences, and other serial publications which get indexed. There are thesauri to build, there are online e-indexes using html and a whole pile of other things I currently know nothing at all about.
There is specialist software available these days to do this which makes life soooo much easier. I did a bit of this 25 or so years ago when it was all 5x3 cards, hand sorted into order, which is really slow and the possibility for getting the order slightly wrong is great.
So I shall get back to it now.