They are explained when you go to the Japanese Lessons.
To get definitions, you use the dictionary and learn words. Conjugations only change the tenses of the word. Through the lessons, it teaches you what the tenses mean and how they're used.
It also may help if you go to other sources that teach grammar if you don't fully understand the depths of the subject. Otherwise, it's still explained on Renshuu. You just may be having a hard time comprehending and remembering things.
You need to know the words in advance on order to answer the grammar questions. If you're using the renshuu-recommended curriculum (Japanese Basics) and are getting grammar/conjugation questions with words you don't know, that means you've skipped too far ahead in grammar and need to finish learning the words in the companion vocabulary schedule (Words for Japanese Basics); the companion schedule will teach you all of the words used in the grammar questions.
