A more thorough explanation:

A letter of attornment is a document that a landlord sends to their tenant, informing them that the property they are renting has been sold to a new owner. The letter directs the tenant to pay rent to the new owner instead of the previous landlord.

Attornment is the tenant's agreement to recognize the new owner as their landlord and continue paying rent to them. This agreement is necessary because the tenant's lease agreement was with the previous owner, and the new owner has no legal relationship with the tenant until they agree to attorn.

For example, if a tenant is renting a house from John, and John sells the house to Jane, John would send a letter of attornment to the tenant. The letter would inform the tenant that Jane is the new owner and that they should start paying rent to her. The tenant would then need to sign an agreement to attorn to Jane, recognizing her as their new landlord.

Another example of attornment is when a bailee, such as a warehouse owner, acknowledges that they will hold goods on behalf of someone other than the original owner. This type of attornment is a form of constructive delivery, where the mediate possessor of a thing delivers it by procuring the immediate possessor to agree with the transferee to hold it for the future on their account.