The expression "a < b" is a mathematical notation that indicates that the value of a is less than the value of b. This means that if you were to compare the two values, a would be positioned to the left of b on a number line. For example, if a is 3 and b is 5, then 3 is indeed less than 5. In practical terms, this concept is often used in various fields such as finance, science, and engineering to compare quantities. Understanding this relationship helps in making decisions based on numerical data, such as determining which option is smaller or less significant in a given context.