The short side of an option sells their guarantee (represented by the option contract) to fulfill the share price for 100 shares per contract if the long side decides to exercise the option before expiration. This means that, if the owner exercises their option(s), the short side is obligated to fulfill the order. The fulfillment of this order is called
If we get assigned it will happen automatically overnight – no action necessary. If you were short a call, you will either sell 100 shares that you already own or end up with short shares in place of your call. If you were short a put you will either find 100 new shares or 100 less short shares in place of the put.