Flashcards are a common way of learning, often used for vocabulary for foreign languages. You get pieces of card, and put the say, English word on one side, and the Japanese word on the other side. You then mix them all up and look at them in a random order. You look at one side, and try to remember
the translation - and then you can check it, by turning the card over.
The basic idea of the card system is that there are 2 different things represented, eg, question and answer, word and translation...etc, one on each side of the card.
This idea simply modernises the concept, by using the functionality common in MP3 players - ie, you can store the 2 different things, 1 as the audio in the MP3, and 1 as the MP3 tag data, and you can play them back using the random play (or shuffle) function.
This is particularly useful for learning kanji (the Chinese characters used in Japanese writing), if your MP3 player can display asian characters. Simply record a number of MP3 files, one for each character, with the audio being the pronunciation, or meaning of the character. Then, in the tag data for the file, you put the character. Then, play them back in random order...you hear the pronunciation, and try to remember the character. You can then check if you are right by looking at the character on the display of the MP3 player. Alternatively, you can look at the screen to see the character, and then try to remember the pronunciation or meaning, and check by listening to the audio.
Depending on the functionality of the particular MP3 player, you may be able to represent more than 2 things. For example, my player displays the file name when in stop mode, and the tag data when playing. So, you could store 3 different things, eg, english meaning of the character in the file name, eg, horse.mp3, the actual character in the tag data, and the pronunciation in the audio.
You can store more than 1 thing in the tag data - common things are Title, Artist and Album, which gives you 3 different areas to store info - but, to be effective as a flashcard, you need to be able to switch between displaying each one, not showing all at the same time.
Of course, somebody (with a good voice ! ;) could make all the MP3 files and make them available for others, so that people don't have to keep making their own....