Customizing Lightning pages in App Builder
Key Takeaway
Saving a Lightning page doesn't show it to users: they see nothing new until you activate the page and assign it.
The Lightning App Builder is a point-and-click tool for building custom pages, called Lightning pages, for Lightning Experience and the Salesforce mobile app. You build a page by dragging components into the regions of a template. When the page is ready, you activate it and assign it so that users can see it.
What a Lightning page is
A Lightning page is a custom layout that sits between page layouts and Visualforce pages. You add items to it, as you do with a page layout, but those items are Lightning components, which give you more flexibility.
Each Lightning page starts from a template that you pick when you create the page. The template divides the page into regions and controls how the page displays on each device. You then drop components into those regions. Components are compact, configurable, reusable elements.
The Lightning App Builder customizes only Lightning Experience pages. For Salesforce Classic record pages, you use the page layout editor instead.