An SNDA is a Subordination, Non-Disturbance and Attornment agreement that defines the relationship among lender, landlord, and tenant.
An SNDA is typically requested by lenders. The subordination part makes the lease junior to the mortgage, the non-disturbance part promises the lender will not terminate the tenant's lease if it forecloses so long as the tenant is not in default, and the attornment part obligates the tenant to recognize the lender or new owner as landlord. Tenants usually insist on non-disturbance, and the buyer's lender will often require the SNDA at closing.