You learn words with Core2K by recalling the meaning and reading of the displayed word. However, I advise against using Core 2K, and suggest you do Tango N5+N4 instead, and here’s why:
>Core 2K is significantly harder than Tango N5+N4 because Tango has the sentence on the front, Core 2K has only the word on the front.
>Tango decks work by showing you a sentence at the front, and the piece of information (a meaning and reading) needed to understand the sentence at the back. You click "Good" whenever you understood the sentence, and "Again" when you did not.
>The "N" number in the deck name refers to the Japanese Language Proficiency Test (JLPT), with N5 being the lowest, easiest and N1 being the highest.
>Sentences make recalling vocabulary easier, after all, it is much more close to how we use language in real life as opposed to isolated words.
However, it is important to note that Tango's translations are not literal, and do not represent Japanese structure well, so I guess that's a flaw of Tango. In other cases, it might be utterly wrong.
People have always struggled with Core 2K, and to make the process smoother, I recommend Tango N5+N4 over Core 2K. Especially if you don’t want to do isolated kanji study!