Apple ID verification happens immediately after creating the ID or after updating certain personal information via a verification email. This email contains a unique link that Apple uses to confirm the email address on file belongs to the owner of the ID.
The basis of Apple ID verification lies in confirming ownership of the primary email address on file. During the initial Apple ID registration process, users are prompted to enter a primary email address which receives all communications from Apple. At the end of the registration process, Apple sends a confirmation email to this address to ensure that the person creating the ID also owns that email address.
Clicking the link sent in the email sends a message to the Apple servers that links the account and the email address and verifies the account. However, this link is often time sensitive, which means that after a certain period of time, the link no longer verifies the account. To resend the email, go to the My Apple ID site and sign in. The user is prompted to request a new confirmation email.
In some cases, the email's spam filters intercept the confirmation email and block its delivery to the inbox. Make sure to search all folders within the email box for the verification email.