What you are seeing above is a working half-adder. Said curcuit can add two binary signals. The upper gate is the sum of both inputs and the gate beneath is the carry signal. Carry signals represent the overflow to the next digit of multi-digit addition.

Check out my last entry on gate delay: https://ldjam.com/events/ludum-dare/46/tyson-lee-keep-it-alive-edition/using-gate-delay-creatively

Come tinker and play at: https://ldjam.com/events/ludum-dare/46/tyson-lee-keep-it-alive-edition