Here is the introduction to Jane Austen's Juvenilia in my edition of the Oxford Illustrated Jane Austen, giving a nice overview of the history of these short pieces:

Under this heading are placed Jane Austen's earliest compositions, her childhood writing, covering the period from about 1787 (when she was twelve) to 1793. In the three manuscript notebooks, entitled by Jane Austen herself, there are altogether twenty-seven items, a collection of transcripts which she made over a period of about fifteen or twenty years as a record of her work and for the convenience of reading aloud to the family and friends. The separate manuscripts of the individual items have not survived. Perhaps those originals were destroyed when copies were entered into the notebooks; perhaps they were dispersed among the relatives and friends to whom they were dedicated, later to be lost.

And from Deirdre Le Faye's Jane Austen:

It is also now, following her return from school, that Jane started to write, beginning by composing the various comic short stories or essays that are referred to collectively as her Juvenilia, and of which she subsequently made fair copies in the three manuscript books known simply as "Volume the First, " "Volume the Second," and "Volume the Third." Not all the pieces are dated, but they were composed between 1786 and 1793 and were evidently written for family amusement, to be read aloud round the fireside, or perhaps to be performed as part of the theatrical entertainments, as three little plays have been preserved. [One of which, The Visit, we'll read next week - CS] Some of them have specific dedications to members of the family and so contain what may have been particularly appropriate references or jokes. … It is a tribute to her youthful intelligence that these little squibs, parodies and burlesques are still comic in the eyes of modern readers. They are deliberately composed, with an amazingly mature appreciation for wit and irony, and display already a sense of literary style and choice of language.

I love this image of the family sitting around the fire, and Jane reading her stories to them. And I can hear them too, can't you? You can tell they were meant to be read aloud, and I can just image the laughter she elicited as she read one outrageous thing after another.