"Conditional": The command block should only execute its command if the command block behind it executed successfully
"Unconditional": The command block should ignore the block behind it
Behind it means the side opposite from the direction the command block is facing.
This differs from chain command blocks because:
"Chain blocks are green. These execute only after the command block pointing to it is processed. This differs from the Conditional option in that it detects what is pointing to it, instead of simply behind it. The command blocks pointing to it also need not be chain blocks themselves to trigger the chain."
I hope this helped. Good luck!