The expression "m+n" represents the sum of two variables, m and n. In mathematics, variables are symbols used to represent numbers that can change or vary. When you add m and n together, you find their total value, which can be any number depending on what m and n represent. This concept is fundamental in arithmetic and algebra, where addition is one of the basic operations. Understanding "m+n" helps in solving equations and problems involving unknown quantities. It illustrates how different values can combine to create a new total, which is essential in various fields, including science and finance.