There's no single US stock exchange — there are many (NYSE, Nasdaq, and a dozen others), plus dark pools. The NBBO (National Best Bid and Offer) is the highest bid and lowest ask available across all of them at any moment — the consolidated best price. Reg NMS (Regulation National Market System) is the SEC framework that makes this work.
The order protection rule
Reg NMS's core provision is the "order protection rule": a trade generally can't execute at a price worse than the NBBO. If a better price exists on another exchange, your order must be routed there (or matched). This prevents your broker from filling you at an inferior price when a better one is publicly displayed.
Why it matters to you
The NBBO is the reference price your fills are measured against — it's what "price improvement" is measured from, and it's why payment-for-order-flow wholesalers advertise fills at or better than it. It guarantees a baseline of fairness: you shouldn't be filled worse than the best publicly quoted price at that instant.
The NBBO is the promise that, however fragmented the market is, you get the best displayed price — not whatever one venue felt like.
The nuance
The NBBO only reflects displayed orders on lit exchanges — hidden orders and dark-pool liquidity sit outside it, which is part of why large institutions trade off-exchange. And in fast markets the NBBO can flicker, contributing to slippage. Still, understanding the NBBO is understanding the fairness floor beneath every trade you place.