What’s the meaning of “fee-simple titles”?

In English law, a fee simple (or fee simple absolute) is an estate in land, a form of freehold ownership. It is the way that real estate is owned in common law countries, and is the highest ownership interest possible that can be had in real property.

In certain jurisdictions, including the UK's England and Wales and Scotland, a freehold (also called frank-tenement and franktenement) is the ownership of real property, being the land and all immovable structures attached to such land. This is opposed to a leasehold in which the property reverts to the owner of the land after the lease period has expired.[1] Immovable property includes land and all that naturally goes with it, such as buildings, trees, or underground resources, but not such things as vehicles or livestock (which are movable).

tl;dr

The Spanish were just borrowing land from the crown, the Americans owned their own.