Basically, {{Learnable}} - is the field for the info you are trying to learn (like a foreign word; the template is set up to expect it as an answer by default), {{Definition}} - is what that info is defined with (like the meaning of that word in a known language; this is what shows on a card as a question). {{Audio}} is audio (of a foreign word), and {{Extra}} + {{Extra 2}} are the fields for any other information you might want to put on a card. {{Mems}} and {{Attributes}} are not used in the current version of the templates and exist only as placeholders for the future.
However, the template is made flexible to be customised for any specific use. There are practically no limitations on what information can go in which field, for example, you can put audio into the {{Defintion}} field and it will work just as well, or put an image into {{Extra}} field an so on (the only restriction is that the field expected as an answer should only contain text – obviously, one cannot type an audio file or an image file with a keyboard, but images and audio are also not supported as options for multiple-choice (yet)).
On top of that, you can add, remove, and rename fields as you choose, change directions of testing and make new types of cards for different combinations of answer and question fields. There are several posts in this thread that detail some customization variants: such as adding new fields, adjusting labels, and adding more testing directions, changing question to audio on an existing card, or making new multiple-choice card (look under the relevant spoiler). Those should give a general idea of how each aspect of the template can be adjusted, but if you are looking for something specific, I can provide directions for that as well.