White Square with Upper Right Quadrant

It marks the top-right quarter of a square. Unicode has a small set of four: ◰ marks the upper-left quarter, ◱ the lower-left, ◲ the lower-right and ◳ the upper-right. A matching set of circles (◴ ◵ ◶ ◷) does the same with round shapes. The marked corner is the only thing that tells them apart.

Because the picture is so literal, it is read the way it looks: a position or a corner. In manuals, diagrams and interface descriptions it can stand for "top-right", such as where a window, a page element or a piece of a grid sits. Outside such technical writing, people mostly use it as a neat geometric bullet or decoration.

Someone sent me ◳. Is it a code?

No, it is almost certainly decoration or a corner hint. No chat slang is attached to ◳. In a nickname, a bio or a heading it is there for its clean, techy look. Next to words about layout, a screen or a map, read it as "top right". If it arrives alone and puzzles you, just ask.