https://www.united.com/en/us/fly/travel/inflight/basic-economy.html
Less expensive fare.
No full-size carry-on bag on domestic, and short-haul international, flights. Just one personal item.
No advance seat assignment. Avoid if you want to sit with a traveling companion(s).
Last boarding group boarding, which doesn't really matter since you won't have a large carry-on bag.
Works well for some people to save some money. Not so much for others. Read the restrictions carefully when booking to ensure they won't cause you any problems.
Big advantage to the airline is that the passengers in the last boarding group don't have large carry-on bags so it reduced the number of gate-check bags, particularly at the end of boarding. Helps avoid departure delays.