Kitchen Close Time specifies what time the kitchen closes regardless of what time the store closes. So whatever value you set up in minutes here the system will close ordering that many minutes prior to the store close time as per Hours of Operation. So for example if the Store closes at 6:30 PM and Kitchen Close Time is 30mins then ordering will stop at 6PM.
Control panel > Configuration > Settings > Edit settings.
The following tab will appear on the right side of your screen,
This will give you time to finish all the incomplete orders before closing the restaurant.
How does this work from the customer's standpoint
As per the hours of operation the store is open till 5 PM
Kitchen Close Time is 30 mins
As you can see below the system is closing the online ordering 30 mins prior to closing time. (current time is 4:32 PM in the below screenshot)