What does -∞ mean? (negative infinity?)

You should probably ask this question under the corresponding video to get a more specific answer, instead of the Help Center. However, I'll help!

Infinity is just a concept of endlessness, and can be used to represent numbers going on forever. Negative infinity is the opposite of (positive) infinity, or just negative numbers going on forever.

Oh, OK!  Thanks!  

I don´t know if negative infinity was mentioned in any of the videos; I just saw it on a practice question.  I´ll go back and look.  

*I have no idea why the apostrophe is an accent.  Something is wrong with the keyboard layout...I don´t know.