What is "default server" of the output of "httpd -S"?

Issue

• What is "default server" of the output of "httpd -S"?

Sample log:

# httpd -S
[Thu Apr 21 09:13:00 2016] [warn] NameVirtualHost *:80 has no VirtualHosts
VirtualHost configuration:
wildcard NameVirtualHosts and _default_ servers:
_default_:443          rhel67.example.com (/etc/httpd/conf.d/ssl.conf:74)
*:80                   is a NameVirtualHost
         default server www1.example.com (/etc/httpd/conf.d/test1.conf:2)   <==
         port 80 namevhost www1.example.com (/etc/httpd/conf.d/test1.conf:2)
         port 80 namevhost www2.example.com (/etc/httpd/conf.d/test2.conf:2)
Syntax OK