This versatile field may serve as a journal, extended task description, even program code of virtually any length (100,000 symbols or more).
Like any change, Description is not saved until you close the form – or tap the Save button when Description is expanded (keeps you in the form – convenient when editing a long text).
By default, the list of cards shows first 2 Description lines. This can be configured (per-board and even per-list) to show more lines or lines from the end, or without folding line breaks (like done in this Welcome Board).