What is the purpose of F() and FPSTR() in ESP8266WebServer -> FSBrowser?
The purpose of the PSTR() macro is to configure the string to be used directly from flash memory. The string is then not loaded to dynamic memory at runtime as it would be without PROGMEM specifier set by the macro.
The purpose of the FPSTR() macro is to cast a string to 'dummy' FlashStringHelper type to help compiler choose the right overloaded function if dynamic memory and flash memory version of a function is available. An example would be the print function.
The F() macro combines this two macros. It makes the string a PROGMEM string and casts it to FlashStringHelper.