An estoppel certificate is a short document a tenant signs confirming the facts of their tenancy: the rent amount, lease start and end dates, security deposit held, whether rent is current, and whether any unwritten agreements or landlord defaults exist. The name comes from its legal effect — once signed, the tenant is generally "estopped" from later contradicting what they confirmed. Buyers and lenders request them during due diligence because they verify the rent roll from the one party with no incentive to inflate it.
For a buyer of occupied property, estoppels are where surprises surface: a side agreement for reduced rent in exchange for yard work, a deposit the seller's records missed, or a claimed repair promise that becomes your problem at closing. For landlords, the practical points are to keep lease files clean enough that estoppels are painless, and to know whether your leases obligate tenants to return them within a set time. Whether and how a tenant can be compelled to sign depends on the lease and jurisdiction.
This is general educational information, not legal or tax advice. Rules vary by state and locality and change over time — check your local law and confirm specifics with a qualified professional.