You are insubstantial and incorporeal, phasing through solid objects as if they weren’t there. You can move through walls, floors, ceilings, boulders, Difficult and Very Difficult Terrain, etc. at your normal Speed. You can walk up and down through the air, water, or solid matter as if there were a set of stairs for you to climb, allowing you to ascend and descend at half your normal Move speed. If you have a magical Fly speed (one that doesn’t depend on wings or other interactions with the air for propulsion) you can use that to move normally instead.
Attacks that deal Physical damage pass through you as if you weren’t there, dealing zero damage. Any of your attacks that would normally deal Physical damage similarly pass through other creatures and objects, dealing zero damage. Only non-Physical damage can be dealt by or to you while you are Incorporeal.
Most physical Maneuvers performed by corporeal creatures have no effect on you - an attempt to Shove or Grapple you just passes through you, for instance. Mental Maneuvers such as Demoralize can still have an effect, though. Weapons charged with elemental magic, such as one enchanted with Igniting Weapon or infused with Burning Brand, can still affect you if they are what a creature uses to perform a Maneuver against you. The Warded Armor and Ghostly Weapon enchantments specifically negate some of the effects of this boon.
Being Incorporeal does not replace your need to breathe - you must still be in a place that can supply the necessary oxygen for you to continue to live, and you can suffocate if you phase your head (or whatever breathing apparatus you have) through solid matter and stay there for too long.