the expression "time is on our side" means that you have the advantage of time in some way. Either you have more time than you need to finish something or you only need a little bit more time to finish something and nothing is going to be able to stop you. the other similar phrase is "time is not on our side" which means the opposite. you have a limited amount of time to do what you need to do, often implying that you may not have enough time.