https://learn.microsoft.com/en-us/windows-hardware/drivers/display/multiplane-overlay-support
Read the first paragraph of link. This is my interpretation, when running windowed mode games + apps, etc... it lets the DWM (desktop windows manager) use MPO mode, which is hardware acceleration for the task of blending multiple planes together aka windows.
Most of the issues i have seen with MPO involve flickering. With windows, there is a Z-axis, which means one window can be over another entirely or partially. Now add into the mix that these windows are asynchronously updating their windows, and in the case of a game, the updates can be extremely frequent and fast like 100s per second. The task is to combine a 3D image into a 2D image to display to user, and optimally do not process the stuff not shown to user.
If the process gets this wrong N times per second, and maybe shows a hidden window that was not processed, the user might see black/random square displayed there. Then goes back to normal, the user might perceive this as flickering.
TLDNR; If you notice flickering, disable it, even nvidia has issues with it. Don't worry.
