In a "math structures" class at the community college I'm attending (uses the book Discrete Math by Epp, and is basically a discrete math "light" edition), we've been covering some basic logic.
I've been reading some of the logic questions on here to get used to notation, etc. However, when I came across the question Visualizing Concepts in Mathematical Logic, I didn't understand what the $\vdash$ symbol means.
It's not in Discrete Math by Epp, nor is it in my mom's old logic book from when she went to college.
Wikipedia's Math Symbols page says it means "can be derived from" when used in a logic context. However, that doesn't make any sense in the above question, as there is nothing on the left of the $\vdash$.
So, what does $\vdash$ mean, especially in the context of the question linked above?