The Bars Visibility tab allows for setting various options that control if bars are displayed disabled, enabled or not at all.
The options vary depending on the frame selected.
Common Options
Alert Level - On Combat - While in combat and when health is at or below this setting, the bar will become enabled.
Alert Level - Out of Combat - When out of combat and when health is at or below this setting, the bar will become enabled.
Hide bars over 100 yards - Hide bars for players who are over 100 yards, generally when not in visible in plain sight.
Target Options
When entering combat - Always Show/No Change/Always Hide the target when in combat.
When out of combat include - Only show Friendly - Do not show hostile targets when out of combat.
Target options are only enabled when the target frame is selected.
Focus Options
When entering combat - Always Show/No Change/Always Hide the focus when in combat.
When out of combat - Only show Friendly - Do not show focus when hostile.
Focus options are only enabled when the focus frame is selected.