The expression "5 + 4" represents a simple addition problem in mathematics. In this case, the number 5 is combined with the number 4 to find the total. Addition is one of the basic operations in arithmetic, which is a branch of mathematics dealing with numbers and their relationships. When you add 5 and 4, you count up from 5 four more steps. Starting at 5, you go to 6, then 7, then 8, and finally to 9. Therefore, the result of "5 + 4" is 9. This process illustrates how addition works by combining quantities.