Where to get app Signing key?

Can anyone please introduce me what the app Signing key is all about? Where to get it?

do you mean in Google Play console?
it some service of play store to give you support in case you lost your key or forgot it.
its not mandatory you can ignore it (opt-out) thats what im doing

Oh yes, you got it correctly! What if I opt in? Can it create any problem in future?

if you opt in from that moment you need to deal with key that google play gave you (upload key) and they save your original key (apk) for security cases .
I think that the biggest disadvantage is that this action is irreversible
more info about that: