Zero is when you don't want to be on bad terms with someone, but you don't want to be their friend. You're neutral; acquaintances. The scale measures from 10 to -10, 10 being good friends, -10 being enemies/hate each other's guts.
Mina is so annoying. Autumn doesn't want to be friends with her, but she doesn't want to be mean. She just wants to be zero.