The expression "7 ≡ 3 (mod 4)" means that when you divide 7 by 4, the remainder is the same as when you divide 3 by 4. In this case, both 7 and 3 leave a remainder of 3 when divided by 4. This is a concept from modular arithmetic, which deals with integers and their remainders. In simpler terms, we can think of it as a way to compare numbers based on their remainders. For example, if you have a clock that resets every 4 hours, both 7 o'clock and 3 o'clock point to the same position on the clock face, illustrating the idea of congruence in modular arithmetic.