In this free video tutorial, Pro Tools Expert Team member Julian Rodgers covers the use of Solo Safe in the Mix window to ensure soloing a track doesn't interrupt wanted audio.
He covers two examples by using solo safe on subgroup Auxes and on effects returns. When a track is put in solo, every other track which isn't also in solo gets muted, this is known as an implied mute and is represented by the track's mute button showing as dull orange and greyed out. Solo Safe is achieved by Command - clicking (Control - click on a PC) on the track's solo button. When in this state the safe track won't be subject to any implied mutes and will continue to pass audio.
Watch the free video to see two basic examples of when this might be useful.