These are investment accounts that contain funds that the account holder has already donated (so they are no longer owned by the account holder and are no longer taxable), but the funds have not yet been donated to a charity (they are invested and awaiting a decision to donate).
Traditionally used by high-net-worth individuals, innovative firms and apps like Daffy are making them more mainstream: they make donating, tracking, and compiling tax-related receipts much easier and centralized.