It signifies underlined. It can also be read as italicized, like when you use underlining for hand-written stuff when you can't do italics. Basically emphasized, but a step down from bold, which uses *asterisks* around the word.
The _underline_ and *bold* conventions are from way back in the day when all you had was a single monospaced font (anyone ever use an ADM 3a terminal?) and no formatting options.
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