How does "showpage()" work in this piece of code?

I have been learning Lua from http://www.lua.org/pil/4.3.1.html and they had this piece of code:

if line > MAXLINES then
  showpage()
  line = 0
end

I don't understand what "showpage()" does here. I don't know whether this is just an example of a function that had to be previously defined (and I don't know if you can actually do this with a function) or is it a library I don't know of.