Modular arithmetic is a system of arithmetic for integers, where numbers wrap around after reaching a certain value called the modulus. For example, in mod 5 arithmetic, the numbers 0, 1, 2, 3, and 4 are used, and after 4, the next number is 0. This means that 6 is equivalent to 1, as it wraps around after reaching 5. This concept is useful in various fields, including computer science, cryptography, and number theory. It helps simplify calculations and solve problems involving cycles, such as determining days of the week or working with clock times.