The kana trainer helps you to take the first step from the latin transcription (roumaji) to native Japanese. Simply select the kana characters you want to see on your cards to replace the roumaji (Jlab’s romanization can be mapped to kana 1:1).
The kanji trainer works in a similar way, selection of the learned kanji is different though. A Japanese kanji character has multiple readings and a unique meaning associated. First, you learn radicals, stroke order and meaning of a kanji using Heisig’s “Remembering the Kanji (RtK)”. A deck for this book is found here. In the kanji trainer, you can then specify the number of your current RtK-kanji and the way kanji should be displayed. If you use furigana, the kanji trainer will always display all kanji with furigana and hide the readings for those kanji you already learned. If you use the jlab representation, the addon will display everything in plain kana and mix in the kanji you already learned.