Notice the icon beside the ID field.
This means the ID field is the table's "key" field.
What is the key field?
A table's key field 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.