What is the meaning of constant term in binomial I'd like to give you an example. Thank you

What is the meaning of constant term in binomial I'd like to give you an example. Thank you

Take binomial (a + BX) ^ n, (a, B are non-zero constants) as an example: (a + BX) ^ n = C (n, 0) · (a ^ n) · (BX) ^ 0 + C (n, 1) · a ^ (n-1) · (BX) ^ 1 + +C(n,r)·a^(n-r)·(bx)^r+… +C (n, n) · a ^ 0 · (BX) ^ n (1) constant term refers to the term whose exponent of variable x is 0