The expression "1 + 2 + 3" represents a simple arithmetic operation involving addition. In this case, the numbers 1, 2, and 3 are combined to find their total. Addition is one of the basic operations in mathematics, often taught in early education to help develop numerical understanding. When you add these numbers together, you start with 1, then add 2, resulting in 3. Next, you add 3 to that total, which gives you 6. Therefore, the sum of 1, 2, and 3 is 6. This process illustrates how addition works in mathematics.