A more thorough explanation:

An official mark put by the post office on an item of mail to cancel the stamp and to indicate the place and date of sending or receipt.

For example, when you send a letter or a package through the mail, the post office will put a postmark on it. This postmark will show the date and location where the item was sent from. It also cancels the stamp so that it cannot be used again.

Another example is when you receive a letter or a package in the mail, it will have a postmark on it as well. This postmark will show the date and location where the item was received.

Postmarks are important because they help to track the movement of mail and ensure that it is delivered to the correct destination. They also provide a way to verify when an item was sent or received.