The antimatter flow converter was a component of the warp drive, involved in the flow of antimatter to the warp core.
Readjusting a fluctuating converter was usually done with a mode stabilizer. The case of a fluctuating antimatter flow converter was part of a demonstration of the Exocomps capabilities for Data and Geordi La Forge in 2369. (TNG: "The Quality of Life")
Also in 2369, the Boslic cargo vessel of Captain Jaheel docked at Deep Space 9, where the readjustment of the ship's flow converter was delayed. Without it, the ship could not go faster than warp one. (DS9: "Babel")