The two finger test, in terms of bra fitting, is how one determine's if a band fits correctly. Put two fingers at the back or front of the bra. If one can fit more, the band might be too loose; if one can't fit two, the band is too tight.
When a band is tight enough, breast weight will be carried by the body instead of the straps.
With a new bra, make sure this is done on the loosest hook.