At any moment there is a highest price someone will pay and a lowest price someone will accept. The gap between them is the spread, and it is a real cost of trading even though nobody invoices you for it.
Buy at the ask and sell immediately at the bid and you lose the spread without the market having moved at all. Liquid pairs have spreads so narrow they are easy to ignore; quiet ones can have spreads wide enough to matter more than any fee. When a single price is quoted to you, it is a simplification of these two.
Binance Academy covers this in more depth: https://www.binance.com/en/academy/glossary/bid-ask-spread
Trading crypto from Dubai since 2019.
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