What does "reply to discovery" mean?

My husband is a cop, and he charged a guy with a DUI, speeding, and failure to comply .... He found out today that the defendants attorney filed a written Reply to Discovery... neither of us hold any idea what that means... a little back?

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In most jurisdictions, a criminal defendant has the right to some kind of discovery from the prosecutor. That is, the prosecution have to provide to the defendant's attorney copies of statements taken, evidence gathered, and other relevant items prior to trial. Depending on the state, the disclosure is sometimes automatic (the prosecutor has to do it whether the defense asks for it or not, within a secure time after charges are formally filed) and sometimes only triggered after the defense makes a formal 'request' for the discovery.
I think what may enjoy happened is that the attorney filed a "Request for Discovery" - maybe he/she titled it incorrectly, or it be mis-typed.
The defendant, in contrast, has no obligation to provide the prosecution beside similar evidence. The prosecution, under the US Constitution, has the burden of proof in a criminal baggage 'beyond a reasonable doubt' and the defendant is presumed innicent at the start of the case.

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Typically a reply is a pleading filed after discovery is received. So by your cross-question it appears that the lawyer has filed a pleading base on what discovery was received. Your question doesn't give satisfactory information, but it may be related to the lawyer believing that he did not receive complete or accurate discovery for some reason.

Your husband could start off by simply calling the attorney to find out what he is looking for. Another alternative may be to ask other officer's what they think, or run the pleading by the department's general counsel.

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