[Android Central Question]

Why and how did it get on my phone

[Rukbat]

Shell is the text input to Linux, which is the operating system that Android runs on. If you learn a little Linux, and install a terminal app, you can use shell (you don't have to type anything - a terminal app is already in the shell). A command like df will show you all the partitions in the phone, and the (usually) last one is the one you use for your storage, so you can see how much space the phone really gives you.

It's the same thing as connecting the phone to a PC, running adb and typing shell. But without the PC.