Admins can set Default On, Default Off and Tab Hidden.

End users can personalise the below actions on Tabs in Navigation Bar

1. ReOrder
2. Add
3. Remove

To Reset, use the Reset Navigation to Default.

Reference Link

https://help.salesforce.com/articleView?id=user_userdisplay_tabs_lex.htm&type=5

When creating a tab, use “Append tab to users’ existing personal customizations”, to override the user’s customization.