The Federal Aviation Administration (FAA) gives each commercial airport a three letter designation. Sometimes, they are very obvious, like MSP stands for Minneapolis St. Paul International Airport, PIT stands for Pittsburgh International Airport, HOU stands for the William P Hobby Airport in Houston, TX, LGA signifies LaGuardia Airport in New York, or LAX means Los Angeles International.
Some are odd, but meaningful, as well. For example, the Stevens Point / Wausau, Wisconsin airport code is CWA. This is a result of the airport's full name - Central Wisconsin Airport.
Some refer to historical names, like Chicago's O'Hare International Airport. It's code is ORD. Orchard Place, IL was a nearby town when aircraft manufacturer Douglas Company opened a plant for making Douglas C-54s during World War II. When Douglas left at the end of the war, "Douglas Field" took the name Orchard Field Airport and the code ORD. Shortly thereafter, the city of Chicago took control of the field and began construction of a new, expanded commercial air terminal.
Port Columbus International Airport, however, perplexed me for a long while. It's designation is CMH. There is a television station in town with the call letters WCMH. CMH is used in a variety of different ways around town. At the airport, I discovered the original name of the airport was Columbus Metropolitan Airport. That solved two thirds of the question.
Finally, I discovered the answer. The H in CMH stands for Hangar. CMH is a VERY old designation for the Columbus Metropolitan Hangar that was a hub for commercial cargo and a few passenger routes in the airport's infancy. As the airport grew, the name didn't.
CMH means Columbus Metropolitan Hangar.