The expression "3 < 5" means that the number 3 is less than the number 5. In mathematics, the symbol "<" is used to indicate that one value is smaller than another. This comparison helps us understand the relationship between different numbers. In this case, since 3 is indeed smaller than 5, the statement is true. This concept of comparing numbers is fundamental in mathematics and is used in various applications, such as arithmetic, algebra, and even in everyday situations like measuring quantities or comparing prices.