Monsters on your side of the field cannot be destroyed in a battle if the monsters' ATK are equal.
Rulings
Individual Card FAQs
If your monster in Defense Position is attacked by a monster with equal ATK to your monster's ATK, your Defense Position monster will not be destroyed even if its DEF was lower than the attacking monster's ATK.
When "Spear Dragon" attacks a Defense Position monster with equal ATK to "Spear Dragon", perform damage calculation normally even though the Defense Position monster is not destroyed.
Spell|
Continuous