A fire shelter is basically a little tent made out of aluminum foil laminated glass cloth which you're supposed to scurry under when you're about to be overtaken by the fire front. When you're on a burn team (or any situation where you could get overtaken by fire) you should carry a fire shelter strapped onto your person for easy deployment. Professionals generally use a special harness which will also hold other important gear like fusees/igniters (basically, flares). Each person should have his/her own shelter, but in a pinch two people can fit in one shelter. These shelters are mostly for grassland fires that burn quickly and don't get too hot. They've been known to simply melt in forest fires where temperatures can get really hot (plus large pieces of burning debris could fall on you).
For a fun walk-through of how to deploy a fire shelter see http://www.ci.sanbruno.ca.us/Fire/Wildfire99/fireshe.html