Selling items for less than they cost to produce is known as "dumping" in international trade (where it is generally disallowed by trade organizations) and can be illegal in the US if the intent is to eliminate competition [0]. That last factor can be hard to prove, and I don't think the FTC is doing much about anticompetitive behavior these days.

[0]: https://www.ftc.gov/advice-guidance/competition-guidance/gui...

To add onto sibling comment: it is specifically when they sell below cost to eliminate competition, with the goal of later being able to raise prices to recover those losses (and more) once they are the only player in town and can jack the prices up all they want. The later price elevations are what result in consumer harm, which is why it is illegal.

Predatory pricing:

A big gorilla comes in and under prices the entire market. They can do that because they already have tons of money. They do this long enough to break the market and drive the competition out of business. Once the competitors are gone they jack up the prices to unprecedented levels because there's no more alternatives available and bleed the market for all the money.

Regular pricing:

Charge a fair price based on actual costs.