For each mission, you are sent to a spot on the map to put out a fire, Once you get there, you aim the firehose and put the fire out with that water. Sometimes the mission is easy - just put out a car fire. Sometimes it's harder, with people in the car who catch on fire and then run around in panic, who you have to put out as well. You get paid for each level of fire.

You're timed in your missions, to put the fire out before the time expires. Be sure not to earn a wanted star during these missions - it's impossible to get away from them in your slow moving vehicle.

level 1 - $100
level 2 - $200

Grand Theft Auto San Andreas Walkthrough