The expression "y_2 - y_1" represents the difference between two values, typically in a mathematical or statistical context. Here, y_2 is the second value, and y_1 is the first value. This calculation helps determine how much y_2 exceeds or falls short of y_1, providing insight into changes or trends. In various applications, such as data analysis or physics, this difference can indicate growth, decline, or variation over time. For example, if y_1 represents a temperature at one time and y_2 at a later time, "y_2 - y_1" shows the temperature change between those two moments.