What does the employee number on the social security card mean?

Yes? The card number of social security card, also called employee number.

After each employee or flexible employee of an enterprise participates in social insurance through the municipal labor and social security department, a number corresponding to the ID card used when participating in the insurance will be automatically generated in the social insurance business system, that is, the number of the employee or flexible employee participating in social insurance, referred to as the social security number, that is, the card number of the social security card, also known as the employee number.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.