These are codes used by the game to call item names, events or symbols. Other codes such as $1 function similarly.
They're also the reason why the batch translation function will simply break games without manual revision, because Translator++ can't tell they're codes. It simply sees text, tries to translate it, and breaks a lot of things in the process.
For example:
%0ダメージを与える。
Should be translated as:
Deal %0 damage.
Where %0 is the code called for the amount of the damage dealt.
But Translator++ doesn't know what %0 means, nor where it should go in the sentence. It often also adds spaces to it, since it can't wrap its head around numbers without spacing, or assumes it's a typo.