dSYM files store the debug symbols for your app.
We've built an insanely scalable architecture on our end to process your dSYM files automatically, end-to-end. You don't even need to worry about it.
The benefit for you is that you don't need to ship your app with debug symbols turned on (which will increase your app's file size ~30% unnecessarily).
Our solution gives you the full benefits without the baggage to your end users! :-)