It means that the variable on that line that you are trying to use like an array (ie., some_array[1]) is actually nil (that is, it has not been assigned a value, or has been explicitely asigned to nil).
Most likely cause is that the array wasn't initialized before you tried to use it, like so:
some_array = []
or
some_array = Array.new
OR, you've made a typo.
