It's creating a list with 2 entries, 0 and the integer value at the row. Then it's indexing that list with the int cast of a == "forward" (I didnt know it would even implicitly cast like this, I hate it!), so if the command is forward, it'll return 1 and index the second value in the list, and if its a different command it'll return 0 and do nothing.

>>> x = [0, 5]
>>> a = "forward"
>>> x[a == "forward"]
5
>>> x[a == "up"]
0
>>> int(a == "forward")
1
>>> int(a == "up")
0