Option expiration refers to the expiration date of an option contract, which is the last date on which the holder of the option may exercise the contract.
Calls and puts give the owner the right to buy or sell a stock at a certain price by a certain date. When the holder of a call or put option has an option that is "in-the-money" (i.e., the strike of a call is below the current stock price, or the strike of a put is above the current stock price) and decides to buy or sell the stock, it is said that he is "exercising" his option.
Option assignment occurs when the buyer of an option decides to exercise, putting the seller of the options contract in obligation to deliver on the contract. For example, if you have sold a call option and the purchaser of the contract exercises, you would be obligated to deliver the underlying shares to the buyer.
Gatsby's policy is to sell out of in-the-money positions on expiration date prior to an option being exercised or assigned. Although Gatsby will use its best efforts to sell such options, Gatsby cannot guarantee that the option will be sold, and therefore cannot guarantee such option will not be exercised or assigned.