If the Lightning app was installed from a managed package, you can append navigation items below the original set of navigation items included in the Lightning app, which are locked.
The locked icon (1) indicates that this set of tabs is fixed and can’t be changed.
Release Notes – https://releasenotes.docs.salesforce.com/en-us/winter18/release-notes/rn_general_lex_app_manager_append_tabs.htm
The locked icon (1) indicates that this set of tabs is fixed and can’t be changed.
Release Notes – https://releasenotes.docs.salesforce.com/en-us/winter18/release-notes/rn_general_lex_app_manager_append_tabs.htm