What Winslow is throwing in the last panel is known as a De'Nuul Pouch, named after the god of Fire (De'Nuul himself had nothing to do with it, they're named as such because they cause a lot of fire). A De'Nuul Pouch could be best compared to a magic time-delayed Maltov Cocktail. The string Winslow pulled activated the magic properties of the pouch, starting a fire that will not spread for a few seconds. The pouch will break upon hitting the ground, spreading a normal non-magic flammable substance in a rather large area. The magic flame, however, will not ignite it for a few seconds (i.e. until the flame can spread from its magically restricted state). Past that, it becomes a wide-area firebomb.

The Landis Royal Army typically calls them flares because it's fewer syllables than De'Nuul Pouch.