The remainder theorem states that if a polynomial p(x) is divided by the linear polynomial x - a, the remainder is equal to p(a). The proof involves writing p(x) as the product of (x - a) and a quotient polynomial q(x), plus a remainder r(x). Since the degree of r(x) must be less than the degree of x - a, which is 1, r(x) must be a constant. Therefore, the remainder is equal to the value of p(x) when x is substituted with a.