The chart itself: the body, the self and the whole of the life. Every other varga is a refinement read against it, and every reading begins here.
Meaning
D1 is the rasi chakra, the chart of the twelve signs: each sign is one part of thirty degrees and the planets are placed in the signs they occupy at birth. It is the root chart, not one division among sixteen, and it is the base against which every other varga is judged.