Complex sentence: National Best Bid and Offer (NBBO) is a SEC regulation requiring brokers to guarantee customers the best available ask price when buying securities, and the best available bid price when selling securities.
Split into simpler sentences: National Best Bid and Offer (NBBO) is a term used in SEC regulations. Brokers are required to execute customer trades at the best available ask price when buying securities, and the best available bid price when selling securities.