The expression "3 + 6" represents a simple addition problem in mathematics. In this case, the number 3 is combined with the number 6 to find the total. Addition is one of the basic operations in arithmetic, which is a branch of mathematics that deals with numbers and their relationships. When you add 3 and 6, you count up from 3 by 6 more. This results in a total of 9. Therefore, the solution to the equation "3 + 6" is 9, which can be verified by counting or using a calculator.