The Low-Income Subsidy is a Medicare program to help people with limited income and resources pay Medicare drug coverage (Part D) premiums, deductibles, coinsurance, and other costs.
You also won’t have to pay a Part D late enrollment penalty while you get Extra Help.
Some people qualify for Extra Help automatically, and other people have to apply.

References

https://www.cms.gov/medicare/enrollment-renewal/part-d-plans/low-income-subsidy