What does it do? for i=1,select('#',...)

I encountered the following code in a project that I'm working with. i don't understand the iteration part of for-loop. What is the select() function?

function _log (str,...)
  local LOG="/tmp/log.web"
  for i=1,select('#',...) do
    str= str.."\t"..tostring( select(i,...) )
  end
os.execute("echo \"".. str .."\" \>\> " .. LOG )
end