Card visibility is a setting that allows you to either prioritize or hide cards in certain contexts and views.
This setting can be found under any card's ellipsis (...) menu.
Currently, there are three visibility settings: Priority, Visible, and Invisible.
This is the default. A visible card will generally just show up where you expect.
This is reserved for cards that are important and they will appear in a few more places than is normal. Notably, this is the Outline (the hierarchy of cards on the left of the screen), where only Priority cards are shown.
This setting allows you to hide a card from most views. However, it is not the same as deleting a card, so all you need to do to see invisible cards is disable the Invisible filter in the filters panel. Invisible cards with never show up in the Command Prompt's global search feature.
New Card Visibility Preference
You can modify what the default visibility of new cards is within Settings > App Preferences > Customize Behavior.
If you're writing lots of cards and finding your Outline is becoming a little too cluttered you can change the default Visibility to Visible, and then manually change cards to be Priority.