Overlap

It pictures overlapping windows, and looks like Windows' restore button. Unicode names it Overlap and describes it as overlapping offset windows. If you use a Windows PC you have probably clicked its look-alike: the middle button in the top-right corner of a window switches between one big square (maximize, 🗖) and two overlapping squares (restore, 🗗). The two offset windows suggest a window that no longer fills the screen and sits over the desktop or over other windows.

Because of that, it turns up where people write about or imitate interfaces without using pictures: software manuals and help texts ("click 🗗 to restore the window"), screenshots described in plain text, interface prototypes built from characters, and programmers' code that draws title-bar buttons from a font. Outside those settings it is rare, and sent in a chat it would likely puzzle people.

The character comes from Webdings, the symbol font Microsoft released in 1997 for web graphics and interface navigation. Webdings held its window icons, this one labelled 'tile windows', as ordinary letters, which meant the text only made sense in that font. In 2014 Unicode added the Wingdings and Webdings symbols as real characters so such documents could be stored and exchanged correctly, and 🗗 was among them. It is a plain text symbol, not an emoji, so it normally appears in one colour like the text around it.