You will have to reply as explained in the letter you received.
Here is some information from Social Security.
https://www.ssa.gov/ssi/text-eligibility-ussi.htm
https://www.ssa.gov/ssi/text-income-ussi.htm
Income that counts for SSI includes free room and board. Having more income reduces your benefit. Income does not include money that someone else pays toward your expenses for items other than food or shelter.
For income tax purposes, someone not related to you can claim you as a dependent if you had less than $4050 of taxable income (for 2018) and paid more than half your total living expenses.
So social security probably wants to know exactly how your roommate was able to contribute more than half your total financial support. If your roommate paid your share of food and rent, that will count as income and could reduce your SSI benefit. If your roommate paid for other expenses, such as medical bills and transportation, that would not count as income that might reduce your SSI.
