Atomicity means that a group of related changes is treated as one whole: all of them happen, or none of them do. It is one property that database transactions can provide.
Imagine a package with several parts that must be delivered together. If one part cannot be delivered, the package should not be marked complete.
For example, when a purchase records an order and reduces stock, both changes should agree. Atomicity helps avoid a half-finished update, but the app still needs to make sure the requested information is valid.