How can i explain the brony fandom to parents.?

You can't: There's no explaining tastes, or explaining any teenaged fandom to the previous generation. Perhaps "Chicks dig it" will be sufficient. Or "It's an elaborate joke -- didn't your generation have something like it?" Your parents have certain expectations for your behavior as a male, right or wrong. Introduce them to a few fellow-bronies who exhibit these behaviors. Don't get over-absorbed. Cheers.