What does human social security mean?

This means that there is no unit that can pay social security for it, and the government is not allowed to pay it in the name of individuals, so find a affiliated unit (or a human resources company that provides such services) to pay it on its behalf.

The difference between it and employee's salary:

Company payment: both companies and individuals have to bear it.

Remittance from human resources companies: the amount paid by companies and individuals is borne by individuals, and the service fee is also paid to human resources companies.

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