We understand there will be times when you need to work on your store and during the process it is not ready for customers browsing. By changing store status to 'Under maintenance', you can easily let your customers know that your store is away but it is just temporary, so that they will check back soon.
1. Go to Settings > General
2. Change Store status
In operation = Your store front can be access in this status.
Under maintenance = Your store front cannot access in this status.
Hope this helps. :)
Attached GIF for extra reference: