An isophote here means a line of equal sunrise (or sunset) time. Every point on one line sees the Sun's upper edge touch a flat horizon at the same clock time on that date.
Around the equinoxes the terminator, the edge between day and night, runs almost north–south, so the lines are nearly straight up and down and sunrise simply gets later by 4 minutes for each degree you go west. Near the June solstice the terminator is tilted, so the lines lean: places further north see an earlier sunrise in June and a later one in December. That is why in June, Arunachal Pradesh and the north-east see India's earliest sunrise, while in December the earliest sunrise in India is in the Andaman and Nicobar Islands.
Each line is a contour drawn through a grid of sunrise times (every 2° on the world view, every 0.25° on India) computed with the NOAA solar formulas, for the Sun's centre 0.833° below the horizon. Times are good to about a minute; hills and the height of the observer shift real sunrise by a few minutes. The date is the local date at each place, so far from the chosen time zone a line can show a clock time on the previous or next day.