Overall meaning : Remember, O believers, when you implored your Lord to grant you victory over your enemy, and He answered your prayers, promising you—through your prophet—that He would reinforce you with a thousand angels in succession to fight alongside you. By this reinforcement, Allah meant to give you good news of victory and so that your hearts might be at rest at the prospect of the coming of the angels. Victory comes from no one but Allah. Allah is Almighty, All-Wise.
And remember when you were weary and worried about your fate in this battle and He made sleep overtake you, making you feel secure, and He sent down rain upon you from the sky to purify you with it from minor and major ritual impurities, free you from Satan’s evil thoughts, strengthen your hearts and fill them with certainty and fortitude and steady your steps by hardening the loose sand under your feet so that they would not sink into it.