Do you know what unlike terms are? If not, you can find simple examples of unlike terms in the article Like terms.
There are only two terms in this polynomial. That is why it is a binomial.
These polynomials (each consisting of only two terms) are binomials.
Note, this is not a binomial:
Why is it not a binomial? It is not a binomial because it consists of two like terms. We can add like terms:
The 7x is a monomial and not a binomial.
So the sum of two like terms is not a binomial.