When you invest in membership shares of an LLC you are generally taxed as a partner in a partnership. Although the partnership generally is not subject to income tax, you may be liable for tax on your share of the partnership income, whether or not distributed.
Form K-1 is used by a partnership to report your share of the partnership's income, deductions, credits, etc. Include your share(s) on your tax return if a return is required. The partnership files a copy of Form K-1 with the IRS.