Top Square Bracket

It groups the terms underneath it in a mathematical formula. Picture a sum like a+b+c+d where you want to say "these first three belong together": a typesetter draws a flat bracket over a+b+c and often writes a label on top of it, such as the number of terms or what they add up to. ⎴ is that bracket. It belongs to a small family of horizontal brackets used in mathematical layout: the square pair ⎴ ⎵, the round pair ⏜ ⏝, the curly pair ⏞ ⏟ and the tortoise-shell pair ⏠ ⏡. The top member of each pair sits above an expression, the bottom member below it. The square pair has a second job too: Unicode encoded it partly for compatibility with older terminal software, where it marks off a run of text written vertically.

In formula tools that support it, such as MathML or LaTeX, this bracket is "stretchy": it widens to cover the expression it marks, however long. As a plain text character it does nothing of the kind. Pasted into a chat or a document, it is a standalone shape whose width depends on the font, and it does not attach itself over the text beside it. That is why it looks right inside real formula tools and odd almost everywhere else.

Mathematicians reach for the curly overbrace ⏞ more often. The square version can come in handy when braces are already doing another job, when groups nest and a second shape keeps them apart, or simply when a style calls for straight corners.

Will you ever meet ⎴ outside a formula?

Rarely, and then mostly as decoration. Someone may drop ⎴ and ⎵ into a username, a text-art frame or a decorated bio because they look like clean little lines; there they are pure decoration. Fonts draw the character differently, and a font that lacks it shows a box or another placeholder instead, so it is a poor choice for anything that has to display the same on every phone.