A field command was a Starfleet leadership position who operated out "in the field", as opposed to a clerk or bureaucrat who sat behind a desk.
In 2267, James T. Kirk was upset to learn that Commodore Stocker had replaced him as commanding officer of the USS Enterprise following Kirk's hyper-accelerated aging, as Stocker had never held a field command before.
Though Spock attempted to explain that Stocker was the ranking officer aboard the ship, Kirk protested that the commodore was a "chairbound paper-pusher" and ordered Spock to relieve him. (TOS: "The Deadly Years")