seller wants to reserve mineral rights

i’m buying a house and seller wants to reserve the mineral rights to the subject property.
Does this mean that they would have the right to dig and extract? does it mean that this transaction is not fee simple (highest ownership)? how would this affect me as a buyer and why is he doing that? your help is greatly appreciated.

forgot to tell you. I live in California, the property is a single family res, in a residential area, if this helps any

When I was a real estate broker in California I saw this on the title insurance all the time. You should call your title insurance company and ask to speak to the title officer who has your file; talk to the title officer not a clerk. They will explain if this seller’s request is unusual or not.

The wording would also be put on all plots when a new subdivision was created. It is nothing to worry about normally, but if it is something new created by this seller I would want to know why, and what would be the effect on your title.