The German city name Frankfurt isn’t unique. That’s why the cities often have an identifier. The biggest and most well known Frankfurt is located at the river Main and therefore called Frankfurt (Main). Main is also used as description in programming which is where the rest of the joke is coming from.

Might be, so better check like this user did:

Just checked my wife’s laptop. Local account, secure boot off, windows 10. It had a message telling me to setup a microsoft account to ‘finish encrypting the device’. I clicked turn off, and it’s currently decrypting the hard drive. Blech.