You can easily filter & visualize a set of cards. For example: see which tasks are due or get an overview of project cards by status in a kanban layout.
Of course you can add these filters & views ad-hoc but you can also save them as views. In this way you can easily re-visit specific combination of a filter + ordering + visualization.
This is great for reporting and status updates but especially also be useful for workflow. For example: have a view that shows which projects are in progress, a view that displays tasks by priority which you can use to easily re-prioritize your work and a separate view that let's you see which tasks you are currently working on.
Views can also be used in teams to coordinate work. For example: have one view that shows cards that need action from one person. When the card is then updated it is hidden from that view and shown in the "work view" for another person.
So how can you create views:
Tip: most templates also have one or more view already prepared to give you a starting point.