In very lay terms, a K-1 is the partnership equivalent of an employee’s W-2 or a contractor’s 1099. Because a partnership doesn’t generally pay taxes of its own, it issues a form to each of its partners, telling the partners what each of them needs to put on his/her/its own tax return. LLCs taxed as a partnership (the default for multiple-member LLCs) issue K-1s. For more on forming partnerships, here’s an article. Or, if you prefer to be entertained, here’s a video.