Square with lower left diagonal half black

Only what the document around it says. Unicode encodes ⬕ as a shape, not a sign with a defined meaning. When you meet it in a chart, a legend, a schedule or a set of icons, look for the key: whoever made it chose what "half dark" means there, such as a partial state or a split between two options.

Its siblings matter more than any single meaning. Unicode has eight half-black squares spread over two blocks: dark top, bottom, left or right, and one for each diagonal corner. If you are copying one into your own document, check that the dark triangle sits bottom-left; a wrong pick flips your picture, and the characters are different for search and fonts. Not every font draws it, so it may show as an empty box or a blank on some devices.