"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!