Mathematicians write a ≡ b (mod m) to mean a and b have the same remainder when divided by m. Read it as "a is congruent to b modulo m".
Example: 17 ≡ 32 (mod 5) because both leave remainder 2 when divided by 5. Another way to think about it: 17 and 32 differ by 15, which is a multiple of 5.
This notation helps when proving modular properties.