Minecraft wiki entry for Command_Block, associated text: Click the "Conditional/Unconditional"‌[JE only]/"Condition"‌[BE only] button to change the command block's conditional behavior:
"Conditional": A command block in conditional mode executes its command only when the command block behind it has executed successfully ("Success count" is greater than 0).
"Unconditional" (default): A command block in unconditional mode executes its command even if there's a command block behind it that didn't execute its command successfully.
"Behind it" in the sense of opposite to the direction the command block is facing, regardless of chain direction or even if chaining is occurring.