A knock-in is a type of option contract that only becomes active when the price of the underlying asset reaches a predetermined barrier level. Once the price of the underlying asset reaches the barrier level, the option is activated and may be exercised by the holder. Knock-in options are typically cheaper than standard options since they come with a contingency that must be met before they become active. They are often used by traders as a way to hedge against potential losses or to speculate on price movements.