Q: Could someone write me the meaning of "certain" in the following sentence?
A certain George Mills, a secret agent, descended from a carriage.
A: It's one of those words that are easy to understand but hard to define.
The best definition I found in a quick search of OneLook is from the American Heritage Dictionary of the English Language: "named but not previously known or mentioned".
So this sentence means something like "George Mills (whom the reader is meeting for the first time), a secret agent, descended...", but you probably wouldn't write it that way.