Black Octagon

It is just a filled octagon; any meaning comes from where it sits. Like ● or ■, it is a geometric shape character, not a word or an emoji. Writers can use it as a decorative bullet, a marker in a chart legend, or a stand-in for an octagonal object when they want a plain text symbol rather than a coloured picture.

Because it is text, it takes the colour and size of the surrounding type unless someone styles it differently, just like a letter. The flip side is that the font decides whether it shows at all: not every font includes this part of Unicode, so what looks fine on your screen can arrive as an empty box on someone else's phone.

Is ⯄ the stop-sign shape?

Almost, but it is turned the wrong way. In the many countries that use an octagonal stop sign, the sign stands on a flat edge and has a flat top. ⯄ balances on a corner. Unicode keeps both orientations: ⯄ (corner up) and ⯃, the horizontal black octagon with a flat top. If you want the sign itself, red and recognisable, use the 🛑 emoji; if you want a quiet black shape that matches the sign's outline, ⯃ is the better pick.