The Miller-Rabin test is a probabilistic algorithm used to determine if a number is a prime. It works by checking certain properties of the number against randomly chosen bases. If a number passes the test for several bases, it is likely prime, but if it fails for any base, it is definitely composite. This test is particularly useful for large numbers, where traditional methods of primality testing can be too slow. The Miller-Rabin test is efficient and widely used in cryptography, especially in generating large prime numbers for algorithms like RSA.