"in" keyword in Ruby

Can anyone tell me what the "in" keyword is for?

I piped the users "input" (from gets) into a block, so I had
{ |in| in ..doing some shit la la la.. }
Ruby promptly threw a SyntaxError, saying that it wasn't expecting kIN.

I find plenty of documentation saying it is a reserved keyword, but I can't find a single example of what "in" does...