This is relevant when releasing Android apps, this keystore will be used to sign the release bundle or package, you can find examples of how this is used in the Muxable repos, note that we usually add the base64 keystore and the password as Github Action Secrets and then use them with env:
Create the keystore:
keytool -genkey -v -keystore %userprofile%\upload-keystore.jks -storetype JKS -keyalg RSA -keysize 2048 -validity 10000 -alias upload
If you are going to store the keystore in github actions, you can store it as a base64 string, you can use bash or git bash.
base64 -w 0 upload-keystore.jks > encoded.txt