Neutral is a behavioral game mechanic applied to various Mobs in Minecraft.
Unlike Hostile or Passive mobs, Neutral mobs will not attack a player or another mob unless they are attacked themselves. When under attack, a Neutral mob will turn to face their attacker, and they will begin to chase, as well as fight back against their attacker until one of them dies.
It is possible for a Neutral mob to fight another mob that would typically not attack them. This is commonly caused when an arrow, shot by a Skeleton, accidentally hits another mob while the Skeleton is trying to hit a player.
Certain mobs may be Neutral in some situations, and Hostile in others. One example of this are Polar Bears. An adult Polar Bear can be neutral, but it becomes Hostile if one of their cubs is nearby a player or if it's attacked.