It's one more than you'd ordinarily be entitled to on the flight after freebies from credit cards/status, and entitlements if a bag is included on the route.

e.g. I bought a Main Plus bundle on a MIA-LAX flight in January. I have no status or AA credit card, and it's a US domestic route so I have an entitlement to exactly zero bags. Buying the Main Plus bundle means I get one bag (and booze and - hopefully - a premium economy seat if they put a widebody back on the route).