In case you're confused, him saying "1D6", will return a value between 1 and 6 (1 dice rolled with 6 sides).
2D6 would return a value between 2 and 12 (2 6-sided dice rolled)
His D6 and D20 are actual variables he created to store the random value, you could just as easily do:
IF 1D20 > 15 THEN
BEGIN
DMG := 5 + 1D6
HP := HP - DMG
END
IF HP < 1 THEN
DEST := @P200
(Unless the scripting system dislikes comparisons with dice rolls)