“I'm not really a fan of horizontal stripes.”
You're shopping for clothes with your girlfriend. She suggests that you buy a shirt, but you don't like it because of the pattern. You explain why you don't like it.
I'm not really a fan of horizontal stripes.
This is one way to say that you don't like something very much. For example:
I'm not really a fan of eating out all the time.
You can use it to talk about something that you dislike a little bit, or you can say this about something that you really dislike strongly. In the second case, this would be an example of understatement.
This expression has gained popularity in the last 15 years or so.
Horizontal stripes are stripes which go side-to-side.
Stripes that go up-and-down are "vertical" stripes.