I read this in Mere Christianity and thought it explained this concept pretty well:
Everyone who believes in God at all believes that He knows what you and I are going to do tomorrow. But if He knows I am going to do so-and-so, how can I be free to do otherwise? Well, here once again, the difficulty comes from thinking that God is progressing along the Time-line like us: the only difference being that He can see ahead and we cannot. Well, if that were true, if God foresaw our acts, it would be very hard to understand how we could be free not to do them. But suppose God is outside and above the Time-line. In that case, what we call ‘tomorrow’ is visible to Him in just the same way as what we call ‘today’. All the days are ‘Now’ for Him. He does not remember you doing things yesterday; He simply sees you doing them: because, though you have lost yesterday, He has not. He does not ‘foresee’ you doing things tomorrow; He simply sees you doing them; bcause though tomorrow is not yet there for you, it is for Him. You never supposed that your actions at this moment were any less free because God knows what you are doing. Well, He knows your tomorrow’s actions in just the same way – because He is already in tomorrow and can simply watch you. In a sense, He does not know your action till you have done it: but then the moment at which you have done it is already ‘Now’ for Him.