It means that, well, let’s say you forgot to do homework and it’s due today. In that situation, it would be an enemy, because you really need to do it but you can’t, ‘cause you have to get ready. But maybe, it starts to rain really hard. You may have time to do it, and time would be an ally.
Time depends on you because if you do something you need to at first, it’ll be completed. But if you don’t, you’ll face the consequences.
Hope this helps! I’m kind of bad of explaining things so hopefully this makes sense.