This is a ripple-carry adder with two full adders (One for each bit)
The inputs on the side are the two operators
(x+y). These are the two numbers you want to add. These are represented by the two first displays on the bottom section.
The input that is in the section with the logic is the initial carry. It is like adding one to the output. This is not required, it's just there to satisfy the simulator.
EDIT: Put the Initial carry input into the decimal point of the first operand's screen
