Delta is a risk measure used in options trading that tells you how much the option's price (called its premium) will change given a $1 move in the underlying security. So, if you buy a call option with a 30 delta, its price will change by $0.30 if the underlying moves by $1.00. If you want to hedge this directional risk you could sell 30 shares (each equity options contract is worth 100 shares) to become delta neutral. Because of this, delta can also be thought of as the hedge ratio of an option.