Left half block

It draws shapes out of text, half a cell at a time.

In a monospaced font characters share one fixed width, a box called a cell. ▌ paints the left half of that box solid, so a row of them makes a striped bar and, next to █ full blocks, a half step. That is why it turns up at the end of text progress bars such as █████▌ (five and a half cells filled), in command-line tools and in pictures drawn entirely from characters.

Outside the grid it also works as a thick vertical accent before a heading or a name in a profile, like ▌ NEWS. There it is decoration with no special meaning.

Where does this block come from?

From the built-in graphics of early PCs and terminals.

Before screens could easily show pictures, computers drew boxes, bars and shading with special characters. The IBM PC character set (code page 437) is the best-known example: it had the full block █, the half blocks ▌ ▐ ▀ ▄ and three shades, and DOS programs and bulletin-board "ANSI art" (pictures made of coloured text characters) built whole screens from them. Terminals and national character standards had similar sets, and Unicode took them over so that old text would survive conversion, which is why ▌ still sits beside its siblings in the Block Elements range.