Delta is one of the options Greeks and measures how much an option's price changes for every $1 move in the underlying stock. Call deltas range from 0 to +1; put deltas range from 0 to −1.
A put with a delta of −0.30 will increase in value by $0.30 for every $1 the stock falls. But beyond just measuring price sensitivity, Delta is also used as an approximate probability that the option will expire in-the-money (ITM).
For wheel strategy traders, Delta is the single most important metric for selecting strike prices. Most wheel traders sell puts and covered calls in the 0.20–0.35 delta range — representing roughly a 20-35% probability of assignment. Lower delta means safer (less assignment risk) but lower premium. Higher delta means more premium but higher assignment risk.