Need help to understand code optfir.low_pass

Hey there, I’m relatively new to this.

I want to know how does the following code works :

chan_filt_coeffs = optfir.low_pass (1, # gain
usrp_rate, # sampling rate (320e3)
80e3, # passband cutoff
115e3, # stopband cutoff
0.1, # passband ripple
60) # stopband

I want to know how the above values for passband cutoff, stopband
cutoff, passband ripple and stopband come about?

Also, what is the difference between using the above code or using the
“gr.firdes.low_pass()” code?

Please help me out.

Thanks very much.