Torque converter is a viscous coupling that takes the place of a clutch in a manual transmission, allowing the vehicle to stop without stalling. In 4th and 5th (maybe 3rd, I'm not sure), the torque converter can lock the input and output together, greatly increasing efficiency and lowering the transmission fluid temperature. Imagine the torque converter unlocked like a slightly slipping clutch.
You can tell if it is locked or unlocked by the engine RPMs. Driving on flat surface, 5th gear, look at RPMs, probably around 2000. start going up a grade and RPMs will increase 2-300 RPMs, that is the torque converter unlocked. If it goes up more than a few hundred RPMs, that is a downshift to 4th.
You can also see the phenomenon by being in sport mode in 5th, and downshifting to 4th. The RPMs will jump, then in a second or two, the RPMs will fall a few hundred RPMs as the torque converter locks in 4th gear.