A 1993 printer's ornament, still asking for permission to be red

Long before phones had emoji keyboards, this shape already existed in Unicode as U+2764, blandly named "Heavy Black Heart" — a printer's dingbat from 1993, meant to be inked in black like any other typographic ornament. When emoji were folded into the standard, that old character was drafted into service as the romantic red heart, which is why it carries a small technical quirk: on its own it still wants to render as plain black text. The red fill only appears when it's followed by an invisible variation selector (U+FE0F), the modern signal that says "show the colourful emoji version, please."

That's why a heart pasted from one app can arrive black-and-white in another — the selector got stripped somewhere in transit.

As for meaning, this is the default heart, the one people reach for when no other colour is doing specific work. It reads as love in the broad sense: romantic, familial, platonic, or simply "I liked this." Because it's the unmarked choice, it rarely gets over-analysed the way the coloured hearts do — nobody wonders what you meant by a red heart. Which is precisely its usefulness: it's the heart you send when you want the gesture understood instantly and completely, with no subtext to decode.