What is the meaning of "one term is allowed per question" (KudoZ rule 2.1)? 

As KudoZ rule #2.1 explains, including multiple terms for translation in a single KudoZ posting interferes with the process of generating glossary entries.

Unless they share a root or are otherwise related, terms should be posted separately--an asker should not ask how to say "apple" and "banana" in the same question. This means that a question such as "flotsam/jetsam/derelict" in a context of maritime law would be not valid, since the terms are independent and should posted in three separate questions.

On the other hand, "the flotsam and jetsam of the inner city" would be OK, as the terms flotsam and jetsam are related in a unit of meaning.

Other examples of valid KudoZ questions are: "screen / screening" (they share a root), "Many levels in this game were heavily tuned for smoothness" (terms related as a unit of meaning in a particular context).