What does ci mean in a street address?

CI stands for "Care of" in a street address. It is used when a person or business receives mail or packages at an address that is not their primary residence or business address. For example, if someone lives at 123 Main Street but has a P.O. Box at 456 Elm Street, they would use the following address to receive mail:

[Name]

123 Main Street

c/o 456 Elm Street

[City, State, ZIP Code]