The advanced segment setting of Match Across Journey allows you to look at the criteria you have selected across the user's entire journey, as opposed to what they currently match.
Take a look at the video below to learn more
For example;
You may want to match users who have been on a PDP at any point in their journey, even if they currently are not. For this you would add in the condition 'Page Info - Type - PDP' with match across journey checked.
You may also want to target users who have had high intent to convert, but now have low intent. For this you would add in the condition;
'Intent to Convert - Low' with match across journey un-checked
'Intent to Convert - High' with match across journey checked