Why do most fields (class members) in Android tutorial start with `m`?

This notation comes from AOSP (Android Open Source Project) Code Style Guidelines for Contributors:

Follow Field Naming Conventions

• Non-public, non-static field names start with m.
• Static field names start with s.
• Other fields start with a lower case letter.
• Public static final fields (constants) are ALL_CAPS_WITH_UNDERSCORES.

Note that the linked style guide is for code to be contributed to the Android Open Source Project.

It is not a style guide for the code of individual Android apps.

A lot of coding guide lines use m for 'members' of a class. So when you're programming you can see the difference between local and member variables.