The expression "9 - 4" represents a simple arithmetic operation known as subtraction. In this case, it involves taking the number 4 away from the number 9. Subtraction is one of the four basic operations in mathematics, along with addition, multiplication, and division. When you perform the calculation, you find that 9 minus 4 equals 5. This means that if you start with 9 items and remove 4 of them, you will have 5 items left. Subtraction helps us understand how quantities change when we take some away.