The function gives you the ‘b’ vector for the filter. The ‘a’ vector is 1, since this appears to be a FIR filter design.
To understand how the filter works, use freqz:
figure
freqz([1.0000 -1.8549 1.0000 1.0000 -1.8514 0.9962], 1, 2^16, 44100)
Use the first two arguments with filtfilt.
(I do not have the DSP System Toolbox, so I cannot comment further on your code.)
EDIT — (5 Jul 2020 at 00:18)
I would design the filter this way:
F0 = 2690;
Fs = 44100;
Fn = Fs/2;
fcuts = [2600 [-5 5]+F0 2790];
mags = [10 0 10];
devs = [0.1 0.05 0.1];
[n,Wn,beta,ftype] = kaiserord(fcuts,mags,devs,Fs);
b = fir1(n, Wn, 'stop', kaiser(n+1, beta), 'noscale');
figure
freqz(b, 1, 2^16, Fs)
.
