Manual exposure is what is achieved when a photographer uses a camera in manual mode. Manual mode requires the photographer to make all the decisions that determine an image’s exposure, as opposed to an automatic or semi-automatic mode that makes at least some of the calls for the photography. Manual exposure gives the maximum amount of control to the photographer.
Manual Exposure Explained
(See also, Exposure)
Manual Exposure is when the photographer is in complete control of their three primary camera settings, shutter speed, aperture, and ISO. Oppositely for example, in Shutter Priority the photographer controls only their shutter speed, and the camera uses the built-in light meter to determine the "correct" aperture. (And ISO, if desired)
Manual Exposure and Auto-ISO
Unfortunately, on most modern cameras the "P-S-A-M" exposure mode dials (program, shutter, aperture, and manual) only directly control two camera settings in the exposure triangle. ISO, and auto-ISO, are still controlled by a completely separate system.
Therefore, it is possible to be in Manual Exposure mode and yet still have your exposure be determined automatically, if Auto-ISO is turned on. For practical purposes, however, the general phrase "use manual exposure" still indicates that all three exposure parameters, including ISO, are being controlled manually.