"Negativity" is the quality of someone who criticizes things too much, acts suspicious of people, gets depressed easily, and so on. In other words, "negativity" is bad emotions.
"Negativity" is a noun that's based on the adjective "negative", so you use it like this:
Don't let that kind of negativity into your life.
Negativity spreads like a disease.
I've had enough of your negativity!