Plain English Meaning
It is a two-part minimum on a non-precision approach: the lowest altitude you are allowed to descend to without seeing the runway, and the minimum visibility you need in order to keep going down once you do see it.
Full Definition
On a non-precision approach, MDA-VIS refers to the published Minimum Descent Altitude paired with the required flight visibility for that approach. The MDA is the lowest altitude to which the aircraft may descend during the final approach segment without visual reference to the runway environment, and the VIS value is the minimum flight visibility the pilot must have to legally continue the approach below MDA toward landing.
Context Anchor
Seen in , such as information and the minimums shown in Figure 10-10 of the .