This is essentially a $\therefore$ symbol upside down, and I have seen it in some proofs, not many. What does it mean?
This symbol just means 'because'. If it was facing up, it means 'therefore'.
Kinda feel like this is too short but I guess there's not much to this question.
$\therefore $ You will have seen it next to the conclusions ... it is equivalent to "therefore", "then", "consequently", "in conclusion", "we obtain" and a long etcetera.
Its inverse is $\because $ that it means << because >>.