So the game offers 30 levels.
This achievement simply means to get a total of 60 stars across all the available levels.
You could go for getting 2 stars in all the levels. You get stars mostly by saving zombies. Requirements for getting stars are always listed at the end of the level (after completion).
Helpful video, credit goes to SilentMugger for his video on Youtube.
https://www.youtube.com/watch?app=desktop&v=NsIAXx7tE1Y&...Achievement helps towards ...