dSYM files store the debug symbols for your app
Services like crashlytics uses it to replace the symbols in the crash logs with the appropriate methods names so it will be readable and will make sense.
the benefit of using the dSYM is that you don't need to ship your App with it symbols making it harder to reverse engineer it and also reduce your binary size
in order to use it in order to symbolicate the crash log you need to drag the crash log into the device's device logs in the organizer of the machine that compiled the app binary (a machine that stores the dSYM)
if you have the dSYM but don't have the machine the compiled the app binary follow the instructions in this link in order to install the dSYM into the machine