The expression "1 + 3" represents a simple arithmetic operation known as addition. In this case, the number 1 is combined with the number 3 to find their total. Addition is one of the basic operations in mathematics, used to calculate the sum of two or more numbers. When you add 1 and 3, you count the total amount of items represented by these numbers. Starting with 1, if you add 3 more, you end up with 4. Therefore, the result of "1 + 3" is 4, which is the sum of the two numbers.