The practical effect
When a customer is hurt in the space you lease and sues both you and the landlord, two policies could respond: yours and the landlord's. Primary and non-contributory wording means yours pays first and alone, up to its limit, before the landlord's policy is touched. The landlord's premium and loss history stay clean, which is exactly why the lease asks for it.
Where it lives
On a general liability or auto policy, as an endorsement or as part of a blanket additional insured endorsement. A certificate alone cannot create it. The description of operations on the certificate should cite the form number so the holder can verify it.
In the wallet
The policy card lists blanket endorsements found in your policy forms, including primary and non-contributory. Holder requests that need it are approved instantly when the form exists and routed to the carrier when it does not.