Your torque converter is essentially a hydraulic pump that acts as the link between the engine and the transmission. Various components within the torque converter use vanes, similar to a turbine, to act on the transmission fluid within the unit to create a fluid coupling, as well as provide the fluid pressure that the transmission needs. Failures within the torque converter can cause symptoms similar to a failing transmission, so include it in a diagnosis of suspected transmission problems.
If you have a transmission failure that released metal salt and friction-material particulates, then you should replace the torque converter as well because you can’t fully flush out the debris. The torque converter is a sealed unit, and as such must be serviced or replaced as a unit. The unit is encased in a shell made of two halves that are welded together. Failures of the welds in the outer or inner joint will cause a fluid leak.