its kinda like an undefined variable notice, except its saying the array element is undefined.
given the following array
$array = array(
'foo' => 5,
'bar' => 5
// works, because $array['foo'] exists
// generates undefined index notice
using $array['lenora'] gives the undefined index notice, because there is no 'lenora' element in the array to use.
you can use isset() or array_key_exists() to check if a variable/array element exists. they have slightly different behavior, the php manual explains the difference.