An indexer scans the content of all newsgroups and organizes the data to help you make logical sense of the vast amounts of content in the groups. This is a big accomplishment as Usenet currently consists of 43 Petabytes of data in 103 Billion posts, across over 100k newsgroups. As you would imagine, the server hardware required to do this is not insignificant.
Imagine trying to browse web pages and find something interesting without a search engine. That is the difference that an indexer makes for newsgroups. Instead of having to browse huge newsgroups by hand, they let you search specific groups, all of them at once, or restrict your search to a range of groups, file types, and sizes.
The power this gives the average Usenet user is amazing, and that is why indexers are so popular.
Some indexers stop there, and that is a wonderful help for most people, but there are indexers that go beyond indexing for searching. These indexers try to make sense of the files posted to news groups. They scan it for being obvious SPAM or a virus, then attempt to categorize the file by subject, sub-categorize by file type, and (if it is a video) even determine the video quality and length. Some even let you keep a list of what you want to download and notify you if it thinks something is posted that matches.
It is a lengthy subject, but don't get overwhelmed. Start with a good but simple "search and download" Usenet indexer. Just don't forget, you will need something like Newsbin (which we provide for free) to actually download the files you find listed, because the indexer will give you NZB files that still need to be given to Usenet software. If NZB files are new to you, be sure to read: What is an NZB file?