What is a "Reply to Defence"?

[bulb763]

I'm claiming against a trader who refused to accept a return under the DSRs. The process has reached the stage where the next step is court and I have been sent a letter with the following by the court:

"...serve a reply to defence setting out the legal basis (quoting sections of the relevant law / regulation) on which he intends to reply."

What exactly does this mean I have to do? They have sent no guidance at all and googling doesn't help either.

[bulb763]

Thanks chaps