A Put Option gives the holder the right to sell the underlying asset. For example the holder of a European put option on a stock would have the right to sell that stock at the agreed strike price on the expiry date of the option. In contrast a Call Option gives the holder the right to buy an asset at a pre-agreed price.
Put options with identical terms to a call option are typically worth slightly less as the maximum payoff that can be achieved from a put is if the asset price goes to zero (the payoff is the strike price less the price at expiry), whereas a call option's payoff is theoretically infinite as the asset price is unbounded on the upside (the payoff for a call is the asset price at expiry less the strike price).
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