Please help me Open Source Air Particle Counter Answered

please help me

I'm making this project:https://www.instructables.com/id/How-to-Build-a-Po...

the part of the circuit is already ready, I have difficulty understanding the program (you can also explain it to me with a flowchart or simply in words)

- I did not understand the software how it counts and divides the particle size?
-pin c6 (mosi) what function does it perform?

- the frequency of the clok and when it is given

from luigi from italy