This means that VLC doesn't recognize the format of one of the data streams in your MKV file. It can't play this file and considers it unfixable.
Without more details, we can't tell you why VLC thinks the file is broken. You can try:
- playing it in another media player;
- updating to a later (or earlier) version of VLC and trying again;
- re-encoding the file (if you encoded the MKV yourself), possibly using different settings or a different encoder; or
- downloading the file again (if you downloaded it from the Internet).
You might look into MKVtoolnix (for Linux, OSX, Cygwin and Windows) or MKVtools (for OSX). MKVtoolnix is a set of tools for creating, altering, and inspecting MKV files; your MKV may be reparable using them. Start with the following command, and inspect the output for errors; it may give you more information about what's wrong with the file: