Extension cords are rated for certain levels of resistive load based on their length. It typically has to do with the gauge of the wire used in the chord itself. Electrical resistance is typically converted into heat energy. If you daisy chain extension cords and have them plugged into appliances, then the resistive load of the chained cords can be higher than the rating of one or more of the extension cords. This can cause fire.
Nailed it in far fewer words than me.