A Tab is a navigation entry inside an App — clicking it takes you to a list view, a Visualforce page, a Lightning component, or a Web Tab (an external URL embedded inline). Each Object you want users to navigate to via the nav bar needs a Tab.
Types of tabs:
- Standard Object Tabs — auto-created for standard objects like Accounts, Contacts.
- Custom Object Tabs — created for each custom object you want navigable. Setup -> Tabs -> New Custom Object Tab.
- Web Tabs — embed an external website inside the Salesforce frame.
- Visualforce Tabs — display a Visualforce page.
- Lightning Component Tabs — display a Lightning Web Component.
- Lightning Page Tabs — display a Lightning App Page (a custom dashboard-style page).
Creating one: pick the type, give it a label and tab style (icon + colour), choose default profile visibility (Default On / Default Off / Tab Hidden), and finally pick which Custom Apps include it.
Permission to actually see a tab is controlled by Tab Settings on profiles and permission sets. A tab can be hidden by default but permitted via permission set, which is the recommended pattern.
In Lightning Experience, tabs appear in the nav bar at the top of the app; users can also pin custom tabs.