They are a buyer of mineral rights. The 12.5% (1/8th) refers to the royalty interest which is probably from an old lease. Many mineral buyers buy ahead of the bit if they think there will be a good chance of more drilling in the near future. Some buyers just want current production. Most only pay for current production with a discount factor for the time value of money. They don’t pay for future royalties because that is where they (or you) will make their profit.
If you just inherited, then learn what you own, if you are in pay status on any wells and what the future potential might be. In my experience, generic letters to buy have been well below the actual value of my minerals.
I moved your question over to Weld County where CO folks may be able to help you better. If you post your location, they can help you with websites and potential activity in the area.