If it's typical, you'll find easy and logical explanations for it. If it isn't typical, ask if the seller has any engineering reports on the issue that you can study, and certainly, if you buy the place, make your offer subject to you having a civil engineer check this foundation.
It doesn't necessarily mean there's something wrong with it, it just means that it's not a perfect rectangle or square shape. Most likely it has some corners and bumped out sections. If the foundation was a perfect square or rectangle the listing agent might put: 24' x 42' in for the size. If it is more complicated the listing agent usually puts "irregular".
Hope that helps! Good luck with the house hunting. It's a great time to be buying with the rates as low as they are and the amount of inventory to choose from. Welcome aboard - Trulia Voices is a good place to get your questions answered.
Unless a foundation (or room for that matter) is a perfect rectangle or a square (I know, same thing) many will default to 'irregular'. Some however, like myself, may state the foundation size in square feet (if it is available and documentable).
Either way, an 'irregular' foundation may be perfectly fine. On the other hand, it may "really" be irregular - In which case, I hope you have a good home inspector. ;)