In layman’s terms, Free Float tells you if an activity is “bumped up” to its successor activity (FF=0) or if there is a delay between the activity and its successor (FF>0). Use Free Float to help you understand if you have flexibility to delay / extend an activity without delaying its successor. You can bump right up to it. This is valuable in manual resource leveling where you may have to delay work if your resources are already busy at that time. You can check Free Float to see how far you can delay the work.
