Access will insert an ID field—the Key field—in the table:
What is the key field?
When the alert window popped up, and you clicked the button, Access added the ID field to the table.
The ID field is now the table's primary key, or key field. That means it can't contain any duplicates.
Every table should have a key field.
For example, if a hospital keeps a database, each patient can have a unique ID number in the key field.
That way, if it has more than one patient named John Baker, it can easily distinguish John Baker, ID #326 in for a checkup, from John Baker, ID #298 who needs his gall bladder removed.