Put
A contract giving its holder the right, but not the obligation, to sell the underlying at the strike, on or before expiration depending on the exercise style.
The buyer of a put pays a premium for the right to sell the underlying at the strike. The seller collects that premium and takes on the obligation to buy the shares if the put is exercised.
A put gains intrinsic value as the underlying falls below the strike. Puts are used to express a bearish view and also to hedge stock that is already owned, so a large put purchase on the tape is not, on its own, evidence of a bearish bet.
Illustrative example (hypothetical numbers, not a real trade): a $50 strike put costs $2.00, or $200 per contract. If the stock is at $44 at expiration, the put is worth $6.00, or $600. If the stock is at or above $50, the put expires worthless and the buyer loses the $200 paid.