Plain English Meaning
It is the lowest height you are allowed to fly on that part of the route when flying on instruments. Going below it is not permitted.
Full Definition
MINDO is a notation found on IFR en route charts indicating the lowest altitude at which an aircraft may legally operate under instrument flight rules along a specific route segment. It accounts for terrain and obstacle clearance, navigation signal reception, and any applicable communication coverage requirements for that segment.
Context Anchor
Seen on IFR charts, instrument procedures, and ATC clearances where a route or procedure is described by named points.