What does "'sup" mean? Is it slang?

Native speaker’s answer
Rebecca
"'Sup" is slang, that's right! It's the shortened form of the informal phrase "What's up," which means the same thing as "how are you doing?" or "what's happening?" as a friendly greeting. Ex: 'Sup, everybody? How was your weekend? Ex: Hey, 'sup? Whatcha been doing today?