Saving Data on Android

How security rules work?
Security rules check the requests that are coming to the database and lets through 
those that satisfy the criteria and reject the ones that don't. So for example, if your 
database only allows writing data to the authenticated client and an unauthenticated 
user tries to write something to the database, then that request would be rejected.
Any request that comes to the database involves the document. You're either trying 
to write the document to the database, read the document from the database, update
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an existing document, or something similar. Cloud Firestore will take a look at the 
security rules that apply to the document that you request contains. It will then run 
a set of tests that you wrote to determine if the request is allowed or not.
In a nutshell, security rules consist of two things:
1. Specifying which documents you are securing.
2. What logic you're using to secure them.