The comma operator evaluates all its operands and yields the value of the last one. So basically whichever condition you write first, it will be disregarded, and the second one will be significant only.
for (i = 0; j >= 0, i <= 5; i++)
is thus equivalent with
for (i = 0; i <= 5; i++)
which may or may not be what the author of the code intended, depending on his intents – I hope this is not production code, because if the programmer having written this wanted to express an AND relation between the conditions, then this is incorrect and the && operator should have been used instead.