Error 1062 (New to this)

I try to insert a .sql and I get this "SQL Error (1062): Duplicate entry '12345' for key 1." Im new to this so a simple explanation would be nice.
Thanks

The .sql contains INSERT statements. A data record provided with the INSERT contains a value for a keyfield (12345); however, a record with this particular key value is already present in the appropriate table of your database. Since the key seems to be defined as unique, it is not possible to insert more than one record with this key.