Fan-donated Love is the Love donated by your Fans while you're streaming, which originates from Love Stickers, a custom Chat Message, or Song Requests. This is in contrast to Bonus Love, which is Love earned by participating in Sessions activities and is added directly by Sessions.
Think of Fan-donated Love as the monetization of the support your Fans, friends, and followers are showing you. See also: Help Article: "What is Love and how do I use it?".