You already expanded a link to get to this card!
Hubcards are supposed to be short (thus the name), however they can link to other cards in order to provide more information.
Linked cards expand directly below the current card, making it easily to browse around without losing your place.
Some cards are "sub-cards" of other cards, meaning that they provide a summary of a sub-topic of that cards topic. This card's topic is "Expand Links" which is a sub-topic of "Tutorial".
Next: Share Cards