This is when you make a hop addition to teh runnings going into the kettle, or to the wort before you've turned on the heat.
There is a rule of thumb for this, IIRC, you take 30% of your late additions and use those for the FWH.
Just toss the hops in the kettle and drain your mash onto them. For extract brews, add the hops when you add the extract (can also while steeping).