Someone or something that is functionally necessary but of small significance or importance within a larger operation or organization. I'm tired of working in this thankless IT support job. I don't want to be just a cog in the wheel anymore!
Also, cog in the machine. One who holds a minor but necessary post in a large organization, as in Frank knew he was just a cog in the wheel of this giant corporation. This term alludes to the role of the mechanical cog, one of the teeth on a wheel or gear that, by engaging other teeth, transmits or receives motion. Used figuratively since about 1930, it sometimes is put as small cog in a large wheel, emphasizing a person's lack of importance.