What do the F and S in the text message mean?

The "S" displayed above the mobile phone indicates the network speed.

Displays the real-time network speed when users use the network. Users can also turn off the display of real-time network speed. Just turn on the phone, find and click "Notification and Status Bar" in the phone settings, and then click the button behind "Show Real-time Internet Speed" in this interface to make it gray, which means that the real-time Internet speed has been turned off. When you return to the desktop, you will find that the "S" is no longer displayed on the top of the phone.