Meaning:

“上辈子” means “previous life,” which presumes that there was a previous life for a human being, “拯救” means “to save,” and “银河系” means “galaxy.” This saying, which can be literally translated as “one must have saved the galaxy in their previous life,” is used jokingly by Chinese netizens to mean that someone has been extremely lucky because of good karma, since they did something extraordinary (like saving the galaxy) in the previous life.

Example:

A: 你家孩子高考成绩出来了吧？

Nǐjiā háizi gāokǎo chéngjì chūlái le ba？

Have you checked the total scores that your son got in the National College Entrance Exams?

B: 他考了全校最高分，应该可以去理想的学校念书了。

Tā kǎo le quánxiào zuìgāofēn，yīnggāi kěyǐ qù lǐxiǎng de xuéxiào niànshū le。

He got the top scores among all students in his school, and will likely go to the university of his choice.

A: 你一定是上辈子拯救了银河系，才生了这么出色的孩子。

Nǐ yīdìng shì shàngbèizi zhěngjiù le yínhéxì，cái shēngle zhème chūsè de háizi。

You must have saved the galaxy in the previous life to have such an excellent kid. (You are so lucky to have such an excellent kid.)