Limit refers to the inbuilt effect of the action. So for
Mordor Warrior, its the reverse of
Trust Me as you Once Did.
From the CRD
When a card has a limit, such as "(limit +3)," the limit applies to that card for an entire phase.
Specifically,
Mordor Warrior can only make 1 companion -3 per skirmish phase. If you can only spot 1 minion, you can discard 3 cards to make that companion -3. If you can spot 2, the first card would be -2, then the second card would only be -1.
Hope this is clear enough.