If you have a Fire creature and put it on a Fire field, it will gain 2HP. However if you summon a Fire creature on a Water Field it will lose 2 HP and be killed if it only has 2HP or less. If you summons a Fire creature on a fire field and the reverse side is a Water field and it is Fieldquaked, then the Fire creature will lose 4HP instead of 2HP.
This applies to every field except for the Middle field which is the Biolith field. The biolith field does not give any monster a HP Bonus. Instead it can be used to summon a Biolith monster with a Summoning Lock on it.