An instant pot comes with a lock so that the lid does not move from its position when the heat is on and the pressure is building inside. Once the food is cooked or even otherwise, you will need to unlock the lid in order to open it.
You will find a small metal valve on the right of the steam release valve. This is called the float valve. When the pressure is up, the valve lifts up as well into its designated place and locks the lid. It is this valve that you will need to open so that you can unlock. You will need to turn it into the previous position so that the lid unlocks.