When you first activate an app page in the Lightning App Builder, a tab is created for the page as part of the activation process. You can also create a tab for the page manually in Setup before you activate it.
Lightning page tabs only facilitate inclusion of the Lightning page in the Salesforce app and Lightning Experience.
You can create a custom tab only for an App Page type of Lightning page.
What Is a Lightning Page Tab?
Lightning Page Tabs let you add Lightning Pages to the mobile app navigation menu.
Lightning Page tabs don’t work like other custom tabs. Once created, they don’t show up on the All Tabs page when you click the Plus icon that appears to the right of your current tabs. Lightning Page tabs also don’t show up in the Available Tabs list when you customize the tabs for your apps.
Steps to create Lightning Page Tab
1. From Setup, enter Tabs in the Quick Find box, then select Tabs.
2. Bottom of the page Click New in the Lightning Page Tabs.
3. Choose a Lightning page for the tab.
4. Enter a Tab label.
5. Select a tab style to set a color scheme and icon
6. Enter a description of the tab, if desired, and click Next.
7. Choose the user profiles for which the new custom tab will be available.
8. Click Save.