The Fire Survival skill is used passively it allows you to access surfaces and areas that might otherwise be inaccessible due to extreme temperatures.
Fire survival is used to access tiles that are very warm, such as fire or lava tiles. Someone without the proper training could easily burn to death or not recognise the dangers such as flashfires and eruptions without this skill so that is why it is required.
See the image below for the list of terrains Fire survival is needed for.
You can also use Air Survival to access such tiles by flying over them.
Additional Information
As with all of the survival skills Fire Survival can be obtained from finding items that contain the Fire Survival bonus on them.
Fire Survival can also be acquired through mounting any of the following pets:
You can also consume certain food items and/or potions to acquire the Fire Survival skill.