what does x reprsent in this code ?

[astral_travel]

well i read about lists and stuff...but the website from which i learned about it was very short in its descriptions of how to do things, it touched everything a bit, in a simplistic way, but it didn't say anything about such a thing. can you elaborate on this ? i'd very much appreciate it,

even direct me to a page where they explain about such possibility...

thanks !