People who live in the southern part of a place, especially the southern part of the United States.
It's just a way to talk about where people are from.
Examples:
The southerners like sweet tea.
The southerners have a different accent.
Southerners are known for their hospitality.
Why do southerners love fried food so much?
Southerners always seem to know how to make good barbecue.
I think southerners are the friendliest people.