It's normal if it's a new Mac or you've just reinstalled the OS - it means it's scanning through everything on your computer so that Spotlight can do it's job. It also does it when you plug in an external drive, it scans all the contents so that Spotlight search will show results for that drive too. Unfortunately it doesn't tell you how long is remaining for the indexing (though on a new Mac it can take a little while).
If this is happening on a Mac that isn't new or hasn't recently had an OS reinstall, then disregard what I said, I have no idea about that