Question about lua socket ftp example.

[albert <albert_200200@...>]

The example code is in the link: 
http://w3.impa.br/~diego/software/luasocket/old/luasocket-2.0-beta/ftp.html
  1 require("ftp")
  2 require("ltn12")
  3 require("url")
  4 function ls(u)
  5     local t= {}
  6     local p = socket.url.parse(u)
  7     p.command = "nlst"
  8     p.sink = ltn12.sink.table(t)
  9     local r, e = socket.ftp.get(p)
 10     return r and table.concat(t), e
 11 end

 I don't understand line 10, Does that mean "return (r and 
table.concat(t)), e"?
 r is a string, Then what does "r and table.concat(t)" mean?