Can someone explain to me the apache log "[info] server seems busy"?

This is the log:

[info] server seems busy, (you may need to

increase StartServers, or Min/MaxSpareServers), spawning 8 children,
there are 17 idle, and 37 total children

[info] server seems busy, (you may need to increase StartServers, or
Min/MaxSpareServers), spawning 8 children, there are 18 idle, and 37
total children

[info] server seems busy,
(you may need to increase StartServers, or Min/MaxSpareServers),
spawning 8 children, there are 18 idle, and 37 total children

[info] server seems busy, (you may need to increase
StartServers, or Min/MaxSpareServers), spawning 8 children, there are
19 idle, and 37 total children

[info]
server seems busy, (you may need to increase StartServers, or
Min/MaxSpareServers), spawning 8 children, there are 18 idle, and 38
total children

[info] server seems busy,
(you may need to increase StartServers, or Min/MaxSpareServers),
spawning 8 children, there are 15 idle, and 39 total children

The message isn't very clear to me and apache2 documentation doesn't help either.

What are those children, why is it spawning 8, what are idle children and what are total children? What is it trying to tell me?