A godsend is something very lucky or helpful — some event or item for which you're incredibly thankful. If you don't have time for breakfast and forget to bring your lunch to school, your friend's extra slice of pizza is a godsend.
If you're grateful for something, you can call it a godsend. A school snow day is a godsend if you're not quite ready for your chemistry midterm, and your homeowner's insurance is a godsend when a huge tree falls on your roof. In both of these examples, you are relieved and happy, and if you're a believer, you might say, "Thank God." In Middle English, it's Godes sonde, "gift from God."