"Lack of Prosecution" on appeal means that the Appellant, (the person who filed the notice of appeal), failed to file appropriate documents in a timely fashion. In this case, this is likely the appellate court's direction to the Appellant to file a response as to why the case should not be dismissed for failing to file the initial brief on time. The "initial brief" is a legal document outlining the facts and arguments in support of the appeal itself, and there are strict time limits that must be adhered to. If you are represented, contact your attorney immediately.
There is quite a bit missing from the facts you presented to give an accurate answer. What this looks like is a docket entry in the court online system about an appeal that is pending. I suggest you call your attorney.
Every case and situation is different and vary greatly depending on specific facts. My posts are not to be considered complete answers to each question. My posts do not constitute an attorney/client relationship.
I agree that you should call your attorney immediately. The word "Brief" is a legal term and is usually anything but "brief". It sounds like your case is in legal peril and you may have other serious problems if you do not respond to the show cause order.
I feel funny even guessing about the meaning where my Florida colleagues won't, but it sounds to me like court clerk's shorthand noting the entry on March 5, 2013, of a court order directing somebody (and I am guessing this must be the person who is taking the appeal) to file a written explanation (i.e. show cause) within ten days why the appeal should not be dismissed for lack of prosecution (i.e., for not filing the initial brief when it was due). You should ask your lawyer about this. A lawyer familiar with the practice in this particular court would know whether this is a normal method of nudging the case along or whether it is a serious danger signal. Be safe and assume the latter.