It looks like a straight angle and can be clicked. It changes shape multiple times and cycles. I see no visible differences when I type something after I clicked it.
This is the tab stop type indicator.
It shows which kind of tab stop will be set if you click in the ruler. In your screen shot, this is a right-aligned tab.
The symbol cycles between left-aligned (common tab stop), right-aligned, center-aligned and decimal point-aligned. The short vertical bar is a clue about alignment.
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