What does "you all" mean?

All of you; plural form of you, including everyone being addressed.

What is a synonym for "you all"?

Common synonyms include: everyone, folks, ningal, you, you are, yourselves.

pron, slang

pron, slang ·Uncommon ·College level

Pronoun

1
All of you; plural form of you, including everyone being addressed.
"I'm very happy to see you all."
2
You (with singular or plural referent); y'all. Southern-US, colloquial, idiomatic
"I am really longing to see you “all”—as the Southerners say—and aren't you coming east this summer?"

"I am really longing to see you “all”—as the Southerners say—and aren't you coming east this summer?"