These macros convert the number of clocks and time(in microsecond).
The clockCyclesPerMicrosecond()/clockCyclesToMicroseconds()/microsecondsToClockCycles() is defined in hardware/arduino/avr/cores/arduino/Arduino.h as below.
The clockCyclesPerMicrosecond() shows the number of clocks a micro second.
F_CPU is (maybe) uses uno.build.f_cpu defined in hardware/arduino/boards.txt. In case of Arduino Uno it is 16000000(=16MHz). As a second is 1000*1000 microseconds, it is 16 which is F_CPU divided by 1000000.
The clockCyclesToMicroseconds() converts the number of clocks to microseconds.
The microsecondsToClockCycles() converts the microseconds to the number of clocks.
June 19, 2020