What is "Slander"? (i.e. "I could sue you for slander")?

I own funny YouTube videos (www.youtube.com/roarcheese12) and a person named Danait Brook shot me an E-mail saying if she wasn't off one of my videos, she could accuse or sue me for "Slander". What is Slander? Now don't see me as a wrong person, I deleted it, I'm just curious...