Delta is one of the greeks that outputs from the option pricing models, such as the Black-Scholes Model. It can be thought of as a bridge that connects an underlying’s price movement with the option’s price movement. Specifically, delta measures how an option will move for a $1 increase in the underlying asset’s price. Deltas for single options are bounded by +1 on the upper limit and -1 on the lower limit, but we typically refer to these as +100 or -100. This can be thought of very similarly to how we communicate percentages as whole numbers, rather than decimals.
Delta essentially offers us two different interpretations: a measure of directional risk and an approximation that the option will expire ITM. Bullish positions will have long (or positive) deltas, and bearish positions will have short (or negative) deltas. It is important to remember that delta should be used in its raw form (sign and magnitude) to determine directional risk, but its absolute value should be used when reading delta as a proxy for probability.