What does fashion mean?

"Old-fashioned" means out of keeping with modern style, outmoded, belonging to an earlier time, not appropriate for the modern era.

People sometimes write it as "old fashion", perhaps because they hear it that way, but it is an adjective, as in:

He drives an old-fashioned car, with huge fins.

The actors wore old-fashioned costumes, complete with bustles.

"You think they are too young to get married? Don't be so old-fashioned!"

The phrase "an old-fashioned look" can mean a stern look at someone who has said or done something wrong.