It refers to when there are 2 or more cards with the same level among all of the stacked cards. Examples include when a level 5 Digimon has a level 5 digivolution card or when a level 6 Digimon has 2 level 5 digivolution cards.

Yes, you can process it. Even if this Digimon's stack doesn't have 2 or more same-level cards, this card's [When Digivolving] effect will return 1 of your opponent's Digimon with 1 or fewer digivolution cards to the bottom of the deck.