Options Greeks: What you should know
The Greeks
Delta – An option’s delta is the rate of change of the price of the option with respect to its underlying security’s price. The delta of an option ranges in value from 0.0 – 1.00 for calls (0 to -1.00 for puts) and reflects the increase or decrease in the price of the option in response to a 1-point movement of the underlying asset price.
Used to measure the change in value of a contract from a $1 change. Also is used to measure the probability of an Option Contract Expiring “ITM” (In-The-Money). For Example, a Delta of 0.40 can be seen as a 40% chance to Expire ITM.
Gamma – An option’s Gamma is a measure of the rate of change of its delta. The gamma of an option is expressed as a percentage and reflects the change in the delta in response to a 1-point movement of the underlying stock price.
Measures the change in Delta from a 1$ movement in the underlying asset (stock, ETF, things like that). If the underlying moves an additional 1$ Then Delta would equal the Total of Delta + Gamma. After the First Dollar move, any additional moves in the same direction increases the value of Delta by the amount of Gamma.  
For Example, XYZ 100 12/31/20 Call for $1.00 and has a delta of .50 and a gamma of .05.
The price of XYZ moves 1 dollar upwards so the new price of the contract becomes 1.50.
The Price of XYZ moves 1 dollar upwards again so now we add both Delta AND Gamma to find the new value. (1.00...