The LSB (A0 and B0) is added by a half adder. The Carry-OUT of the half adder is connected to the Carry-IN of the full adder. The full adder now adds the MSB (A1 and B1).
The result is displayed by Q0 and Q1 with the Carry-OUT CO.
This circuit is called a ripple-carry adder. In the next experiments, you will learn what this is all about and why this circuit has a serious drawback.