Bid, Ask & the Spread
The bid is the highest price a buyer (usually a dealer) will pay; the ask (offer) is the lowest price a seller will accept; the difference is the spread — the dealer's compensation and a gauge of liquidity.
A quote of 20.00–20.05 means you can sell immediately at $20.00 or buy immediately at $20.05. Market makers earn the spread for committing capital; the inside market is the highest bid and lowest ask across all quotes.