How to measure frequency of sine wave?
This is the key element of the frequency measurement code:

TMR0=0; // clear TMR0
Delay_ms(1000); // Delay 1 Sec
...
Display_Freq(TMR0/2); // divide by 2


The author is using the TMR0 timer/counter to count the zero-crossing events of the input signal. (The article uses a transformer to step down 220VACrms 50Hz mains line frequency to 9VACrms, then uses a full bridge rectifier and a resistive divider to generate the zero-crossing event signal. I'm not sure how they intended to test this circuit.)

After running the counter for 1 second, the value in the counter is equal to double the frequency, because each full-wave rectified cycle has two pulses.

The rest of the code in Display_Freq(f) is just user interface code, to convert the numerical frequency into characters to display on the LCD.