Downwards Arrow With Tip Rightwards

It hangs that line under the one above it. The shape draws the link: a line drops from the item above and turns right into the new line, the way an outline or a family tree connects a branch to its parent. So "Trip ideas" followed by "↳ Lisbon" and "↳ Porto" reads as two options under one heading, and "Meeting moved ↳ now Thursday" reads as a detail of the first statement.

The same logic covers its other everyday jobs: marking a reply or comment under a post in notes and documents, showing a result ("Step 3 ↳ save the file"), and leading the eye from a profile bio or caption to the link or detail on the next line.

What does it mean when someone sends it?

A pointer, not a feeling. ↳ carries no tone of its own. In a message it usually says "here's the follow-up" or "this answers the line above". Sent alone, it most likely means "see what comes next" or that a list lost its formatting; ask rather than read a mood into it.