For the List Trigger campaigns, you can choose whether you want to enroll users that already match the criteria you defined for your campaign.
For example, you might want to create a campaign for users who are VIP clients and have spent over $1000 (or more, depending on your shop) in the past month. The campaign will start sending communications from moment you launch it.
Only people who automatically enter the dynamic list after the campaign start will receive these messages. The ones who were included in the list beforehand will no receive them UNLESS you tick the box next to "also enroll the existing users".
Once you've ticked this box, the users already present in your dynamic list + the users who will automatically enter the list will receive your communications.