Prefix codes are numbers and/or characters that are used to withold a phone number / caller id.
They are specific to the country and/or carrier.
When added before the number being dialled, the phone number / caller id from which you are calling will not be shown on the other person's phone.
A list of prefixes codes for specific countries can be seen here (Disabling Caller ID Section).

NOTE: The prefix code is NOT the country code.

Hide My Call automatically prepends the defined prefix to every call made.
Hide My Call includes a database of commonly known prefix codes and will attempt to automatically set that code based on your country and carrier.

View the How Do I Set a Prefix Code Manually support article to learn how to change the prefix code manually.
View the How Do I Update the Prefix Database to learn how to updated the Hide My Call prefix database.