Core terms
Ask (offer)
The lowest price a seller is currently displaying for a stock, together with the number of shares offered. Buying "at the ask" means buying immediately from that displayed seller.
The ask, also called the offer, is a standing offer to sell. The lowest one is the best ask. A market order to buy fills against it first and then climbs to higher asks if more shares are needed, which is how a large market order can pay several prices in one fill.
As with the bid, the display shows only what your feed covers. A thin ask can be replaced by a higher one in a fraction of a second.
What traders should verify
Before buying a fast-moving small cap, look at how many shares sit at the ask and the next few levels, not just the top price.
Do not conflate
An ask can be cancelled at any time before it trades. Quote displays differ by data feed.