The video discusses the concept of a being congruent to b mod n, where a and b have the same remainder when divided by n. It emphasizes the importance of n being a positive whole number greater than one. The video also explains that a is congruent to b mod n implies a is equal to k times n plus b for some unknown k, and when you divide by n, the result is b. Another interpretation is that a - b is a multiple of n. The video highlights the significance of this notation in working with equations rather than congruences. It concludes by stating that n divides into a - b, a common notation used frequently.