Bid
Bid is the highest current price someone is willing to pay in the order book.
Definition
Bid is the highest price currently offered by buyers in the order book. The gap to ask is the bid ask spread.
Why it matters
Bid helps you understand liquidity at the top of book. If bid is far from ask, execution costs are high. Bid is also a conservative reference when evaluating short side comparisons.
Common pitfalls
Confusing bid with last trade: The last trade can be stale.
Assuming depth: A bid quote can be small size and disappear quickly.