Date Last Insured means the date your Social Security Disability insurance lapsed.
SSDI works like an insurance plan. When you stop paying your premiums, your insurance lapses. When you stop working, your Social Security Disability insurance lapses.
To meet the insured status, you must have:
Paid in Social Security taxes over a long period of time to be “fully insured” AND
Paid in Social Security taxes recently enough to have “disability insured status.”