This is very complicated stuff, so I'm not sure if I got it right, but here's how I understand it:
Short answer: it describes that something's in the past/preceding/previous to something else.
Long answer: this article is about the Old Latin usage of future perfect, which in Classical Latin means e.g. "I will have completed". Apparently in Old Latin it was closer to future tense with perfective aspect (which afaik means basically, describing a one-time event, as opposed to imperfective aspect which denotes an indefinite stretch or repetition of an action), as in something like "I will complete [at a certain point of time]".
So Classical Latin "I will have completed" describes a future action, but from a point of view in the even more distant future when it's already past. Thus, the action precedes the future scenario, and hence: "vorzeitig".
Finally, the "perfektisch" part marks a special aspect distinct from imperfective and perfective: it's also called resultative and focusses on the outcome of the action. I'm no linguist, but I think English "I have completed" fits that sense if the focus is on the fact that it is now completed. This is basically what's happening in Classical Latin future perfect: "in the future, it will have been completed".
I ask anyone who understands this all better than I do to correct me.