What does it mean to supplement the old-age insurance on a monthly basis?

Refers to the insured person's unpaid endowment insurance premium, and the social security bureau pays it back.

(For example, if the insured fails to pay the endowment insurance premium due in June due to insufficient amount withheld by the bank, and the insured deposits a sum of money in July, the Social Security Bureau will withhold the unpaid amount in June when withholding the endowment insurance premium in July).

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