Sometimes, when searching for messages in a server, you may run into an "indexing" message before getting your results. There's two possibilities where you'll see this:
First time indexing:
For particularly large servers, we may need an extra minute or two to go back and index older messages. Think of your search results like a specific file cabinet; indexing is your lovely Discord librarian searching the rest of the files (messages) in the archive (your server) and labeling all of them so she can bring the correct ones to your cabinet.
Index all the things:
We'll try our best to index the entire server when you're searching for a message, but for super large servers, it may take a bit longer. You'll get roughly the first two weeks of messages in this case, and you'll see the indexing message continue at the top of the search tray. While you can view the initial messages in the results, the rest of the server will still be indexing. When it's done, you can fast forward through the extra pages at the bottom and have all the results ready to go!