CardView is a widget for implementing 'card-styled' views in Android.
In Android, a card is a self-contained view similar to Android's FrameLayout (in fact CardView extends FrameLayout) which gives you the possibility of setting up an high contrast between the card and the background through elevation, thus generating more immersive apps.
CardView is often used in combination to RecyclerView. Usually, you would have multiple cards and use the RecyclerView as a container for scrolling between them.
You can read more about cards and recycler view here: https://developer.android.com/training/material/lists-cards.html