Basically it is up to the chip creators as to what exactly this means. To one tuner, Stage 1 could mean a 50hp boost, while to another it could mean a 35hp boost. There isn't going to be one set answer for any given chip. The only thing which will usually hold true is Stage 2 will give you a bigger boost than a Stage 1 will. A Stage 3 will give you more than the Stage 2 and so on. You cannot compare a Stage 2 chip from Tuner A and expect it to perform better than a Stage 1 from Tuner B. You have to compare what they say it will give you in direct power/torque boost numbers.
With any chip, you have to ensure you are doing your due diligence, reading about your proposed mods and ensuring it will give you the performance aspects you are looking to add for your vehicle.
When doing the research, you'll also want to read about how customers liked the chips to ensure you are getting what you paid for. Most tuners will exaggerate the gains seen from their chips, to include fuel mileage, etc. Good tuners will give you what you're asking for and you'll never have an issue with them. A custom tune may be a more likeable solution, considering you'll get exactly what you are looking for and the tune is done on a dyno. This method is a little bit more expensive, especially with a chip, but well worth it in the end for the product you're given. The reason doing a custom chip tune is a little bit more expensive is because it takes specialized equipment (chip burner and eprom chip) to finalize the tune. You don't find this very often with tuners because they are used to doing tunes for newer vehicle where tunes are written directly to the ECU.