What is the benefit of utilizing a serial number?
A serial number provides a unique and permanent identification of a record. Two people can have the same name - but their records have different serial numbers. A person can change their name - but a serial number is forever.
What is the primary function of a serial number vis-a-vis a database?
To link between records in different tables. For example, People can have Things assigned to them. In the Things table, the assignment is done by entering the person's serial number into a field. This way, the thing will still be assigned to the same person even if they do change their name (or any other attribute).
How do I assign a serial number?
By defining a field (usually of type Number) with auto-entered serial number. Since you have already imported some records, you can place the field on the table's layout, click in it and select Replace Field Contents… from the Records menu. Then choose to replace with serial numbers + 'Update serial number in Entry Options' (so that new records pick up from where the replace left off).