Hi @Gayane_Ghevondyan,
thanks for reaching out! Here you’re expressing your background as a polynomial b(x)=a0+a1*x+a2*x^2+...+a_n*x^n, so if you give a list of length n you’ll get a polynomial background of degree n-1. If you specify two parameters you’ll get a line and only one parameter will turn out into a constant.
Hope this helps.
Cheers,
Monica