e.g if you created a

/config/lang/en.user.xml

file with the following

<language alias="en" intName="English (US)" localName="English (US)" lcid="" culture="en-US">
    <area alias="dashboardTabs">
        <key alias="Our.Umbraco.TheDashboard">Hello Bob</key>
    </area>    
</language>

then it would change the tab name for the dashboard tab.