Sideways White Right Pointing Index

It means “look here” or “pay attention to what follows.”

A manicule is a pointing hand used to draw attention to a passage; the name comes from the Latin word manicula, meaning “little hand.” Readers drew these marks in manuscript margins to flag noteworthy passages or section openings. Pointing-hand marks appear in surviving records by the twelfth century, grew more common in Italian books in the fourteenth and fifteenth centuries, and later became type offered by printers.

What does it mean when someone sends it?

It tells you to look at the message content it points toward.

Before a link, quote, or instruction, it directs attention to what follows. Alone, it has no named target, so the surrounding message has to supply one. That placement gives it the feel of an editorial attention mark.

Why does the hand look different on some screens?

The active font controls its outline.

Fonts can draw its cuff or hand shape differently. If no available font contains this text character, the screen may show a blank box or replacement mark; the underlying character has not changed.