Millions of Americans own oil and gas mineral rights and have no real idea what that means. Some inherited a folder of yellowed documents from a grandparent's estate. Some bought a piece of land and assumed everything under it came with the deed. Some get a small check in the mail every few months and couldn't explain why if asked. Mineral rights are one of the most valuable and least understood assets in the country, and a whole industry has built itself around that gap in knowledge. Landmen, mineral buyers and self-styled "finders" all make a…