When someone isn't good enough to run with the big dogs (the varsity team) they are just junior varsity.
the state of being average or okay at something.
not quite the best, but good enough.
zach-"how did you do on that physics test Friday?"
stephen- " my score was jv, i'll do better next week"
zach- "how was she last night"
stephen- "dude, she was just jv, I think I can at least get someone who is sitting the varsity bench next week."