No offense, but it seems like you are confused about the very process of learning a new language in general, rather than anything specific about Japanese.
Of course there's no way to know the meaning of a Japanese word before you learn it for the first time. The only way to learn a language is by learning new words, grammatical structures, sentence patterns, etc., memorizing them, practicing them, seeing them in various contexts, etc. etc.
If the app is just "quizzing" you on words you've never seen before, then it's probably just meant for review, and not particularly well-suited for actually
teaching you the language.
I would recommend trying to find a better learning app, or better yet, get yourself an actual textbook designed by professional educators for the purpose of teaching Japanese to non-native learners.
The GENKI series is probably the most highly regarded of these:
Amazon product ASIN 4789014401
(There are also various resources available for free on the internet, but these can often vary widely in quality.)