Medium four pointed black star

It's a plain decorative star with no hidden meaning. In type design, "black" means filled and "white" means outlined, so this is a filled four-pointed star symbol; "medium" is its stroke weight. Unicode gives the light 🟄, medium 🟅 and regular 🟆 versions separate codes so text keeps the exact look of the original font. In a message it does what any star dingbat does: starts a list item, frames a heading or adds a little sparkle. Read the words around it, not the star.

It arrived in Unicode 7.0 together with hundreds of pictures from Wingdings and Webdings, Microsoft's symbol fonts of the early 1990s. In those fonts, typing a letter produced a picture; documents changed meaning the moment the font was missing. Giving each picture its own Unicode code fixed that, which is why this star now exists as a character of its own.

Why do I see a box or a different-looking star?

Few fonts include it, so devices often can't draw it. When no installed font has 🟅, you get an empty box or a question mark, while the sender sees a star. Fonts that do have it draw the arms a little thicker or thinner, and because it's a text symbol, not an emoji, it never turns colourful. If you need a star that shows up almost everywhere, use ✦.