For decades, Southwest's key policy has been that bags fly free. The airline just killed it—and eroded its brand at the same time.

Southwest Airlines will begin charging customers a fee to check bags. It's a significant change for the budget carrier, which built years of advertising campaigns around its policy of letting passengers check up to two bags for free.

The carrier announced Tuesday that it will begin charging certain passengers to check their luggage on flights, a significant shift at the company long prized by consumers for its perks.