Metrical Breve

It means: this syllable is short. Scansion is marking a line's rhythm syllable by syllable. Ancient Greek and Latin poetry built rhythm from long and short syllables, so scholars write a long mark (–) over or under each long one and ⏑ for each short one.

Strings of these marks name the classic feet: the dactyl is – ⏑ ⏑, the iamb ⏑ –, the trochee – ⏑, the anapaest ⏑ ⏑ –. The opening of Homer's and Virgil's epics runs in dactylic hexameter, six feet built mostly from – ⏑ ⏑.

Teachers of English verse often borrow the same cup-shaped mark for an unstressed syllable, against a stress mark for a stressed one. There it means 'weak beat' rather than 'short'.

Why use ⏑ instead of the ordinary breve ˘?

Because it is a standalone sign, not an accent. The breve ˘ is a diacritic meant to sit on a letter (as in Romanian ă). ⏑ sits on the line by itself, at the size of the other metrical signs, so a pattern like – ⏑ ⏑ – lines up neatly. It was encoded with a small set of companion signs for writing these patterns. Many fonts lack it, in which case it shows as an empty box.