Direct Line of Inheritance

What does it mean to have 'a direct line of inheritance? Is that just a term that is used by the lawyers ? What is the purpose of using it?

In Texas, I don't see that term, but I do know that the landmen work to get title from the various courthouses. I do have some title opinions that go from the sovereign to the first owners and down to me. That is the best way to determine your ownership.