The best low possible on that board.
It is the two cards that make the lowest hand with three board cards of 8 or lower. On 3-6-7 it is A-2, on 2-5-7 it is A-3, and on A-2-7 it is 3-4. A hand with only A-2 ends up with the nut low on the river only 25% of the time (43% with a third low card). Treat any low other than the nuts as worth 0.