I think you mean "BIOS" modded. This is just a procedure where someone has taken the original BIOS out of a Neo Geo console and replaced it with an enhanced one.
With the modded BIOS, you can switch Regions (US/Japan/Europe), enter Dip switch codes, and more. If I'm not mistaken, you can switch so that a Japanese game will play in English or vice versa, plus the dip switches will allow you to change certain parameters like number of lives, etc.
Check out the Neo Store...they have the chips in stock. Of course, if you're not a technical person, then you can pay someone to do it install it for you. Well worth doing in my opinion.