Integral extension

It is the stretchable middle of a tall integral sign. The integral sign ∫ is a stretched letter S. In printed maths it grows to match whatever stands beside it: a fraction, a matrix, a stack of lines. Fonts and screens without real maths layout can still build that tall sign from pieces, and ⎮ is the piece that can repeat as often as needed.

Each piece sits on its own line of fixed-width text: ⌠ on top, ⎮ in the middle (one per extra line), ⌡ at the bottom. Programs that print formulas in a terminal or a plain text file use exactly this trick, and so do people writing maths in emails, chats or code comments where nothing else is available.

Why does it look like an ordinary line?

Because alone it is only the straight part. Without ⌠ above and ⌡ below, ⎮ shows as a plain vertical stroke, and some fonts draw it shorter or thinner than the line height, so the stack can show small gaps. A monospaced font usually joins the pieces best.

If you just need an integral in normal writing, use ∫. The pieces matter only when the sign must stretch over several lines of text.