You asked to it be explained in English, so here it is!
Basically, renshuu decides whether or not to show furigana depending on how much kanji you know. Of course, there is no way to perfectly match up your known kanji without telling renshuu in some way what you know. The most natural (but not ideal for everyone) way is to study kanji on renshuu, and any kanji you study will start appearing in sentences/questions without furigana.
You can also, through the Kanji Index or the kanji dictionary, mark any kanji as known using the light bulb icon.
However, if you just want to have the kanji always appear without furigana regardless of the above, you can do that too! If you open up the menu, then tap the settings icon, jump to the Vocabulary section. There is a "What do do with kanji you don't know" setting. Choose the far-right option (show the kanji without any furigana), and this will most likely like you up with what you're looking for.