To start with, delta measures the rate of change in option price for a 1 point change in the underlying price. So essentially, delta measures how sensitive the option price is to changes in the underlying price.
Answering your question, the key factors that affect the delta of an option are:
- The type of option (Call or Put)
- Strike price of the option
- Underlying price
- Time to expiration
- Implied Volatility
We have spoken in detail about delta in FYERS School of Stocks. To read the chapter on Delta, click here59