A bounded, self-contained element within the UI that signifies a tightly related set of information and actions.
A "card" gives information and actions a dedicated piece of real estate within the UI to reinforce their relationship to each other and within the overall hierarchy of the UI.
Cards are the layout equivalent of "objects" in programming. They contain a specific object which can define a specific approach to solving its unique requirements.
Cards are often a summary of something larger, which can be inspected in more detail by interacting with the card itself in some way.
It is common to see unique layouts for each "type" or category of thing represented by a card. For example, an article card will likely have different elements and actions than a video card or calendar event card.