Fiery Dragons and the dreaded Hydras of the Chaos Wastes, as well as some even more bizarre creatures, have the ability to breathe fire, noxious gas, lightning or even stranger substances.
A model with a breath weapon may use it in the shooting phase, even if it has marched. Use the flame template, placing the broad end over your intended target as you wish and the narrow end next to the creature's mouth(s). Note that the monster must be able to see all its potential targets. Any model that lies completely under the template is hit automatically, while models whose bases lie partially under the template are hit on a 4+. The Strength and any special effects of the breath weapon will be detailed in the creature's rules. Characters under the template are eligible forrolls if they are in a unit, as normal.
Breath weapons may not be used as a stand & shoot charge reaction, and neither can they be used when engaged in close combat (and, of course, cannot be targeted at units that are engaged in combat).