They say Amoongus is an excellent pivot, bur I don't quite get the meaning of that.
a pivot is a Pokemon mainly used for switching into. Defensive pivots are pokes with good defenses or a good type against the opposing Pokemon (ex switching into a flying type to counter a ground type). an offensive one is used when you want to try to force your opponenet to switch(ex. switching into a powerful poke that could KO theres).