Summary
Devil Fruitsare the main power system inOne Piece, but as time progressed, there was a need for a counter to these incredibly strong powers, and a way for non-Devil Fruit eaters to put up a fight.
Hinted as early as chapter four in the manga, Haki is the post-time skip power-up that most characters need to overcome their new adversaries. Haki, especially Conqueror’s, is still shrouded in mystery, and it leaves endless opportunities open for amazing battles to come in the long and winding story of everyone’s favorite rubber pirate.

Spoilers ahead for the entirety of One Piece.
What Is Haki?
Similar to Chakra in Naruto, Hamon in Jojo’s Bizzare Adventure, or even Ki from Dragon Ball,Haki is an energy that is found within everyone, and it is up to them to train it properly and become the strongest. Like a muscle, Haki needs to be trained to become stronger, but pushing it to its limits can significantly increase its power.
Haki is something everyone possesses,but not everyone can utilize it. Its use requires intensive training, such as the one that Luffy underwent with Silvers Rayleigh during the Amazon Lily arc.

Different Types Of Haki
Although Haki is the umbrella term for the power system,there are three different typesthat one can possess.
What Is Observation Haki?
Even though Haki is system that is introduced post-timeskip, small hints have been sprinkled throughout the story. InSkypiea, the four priests of Upper Yard all have an ability known asMantra, which is actually Observation Haki.
Observation Haki, or known as Kenbunshoku Haki in the manga,gives its user enhanced senses and perception. Not only can the user, see and hear their surroundings much clearer, but when facing an enemy, they can even sense their opponent’s next move.

Training in this form of Haki can unlock future sight, as seen in Luffy’s fight against Charlotte Katakuri.
In the Loguetown arc, the seemingly untouchable Navy captain, Smoker, is introduced. His ability to turn his body into smoke presented a problem to the Straw Hats since none of them would be able to damage him. Armament Haki solved this problem.

Known as Busoshoku Haki,Armament Haki allows its user to coat any of their attacks with their aura, creating an almost impenetrable armor. This type of Haki is immune to Logia-type Devil Fruits and can even be imbued into weapons.
This type of Haki is the main one used by non-Devil Fruit eaters.

The strongest Haki type by far also happens to be the rarest.Unlike Observation and Armament, mastery of Conqueror’s (Haoshoku) Haki is not a task that can be achieved through training alone.
Conqueror’s Haki is an ability that only a handful of individuals are born with. Legend says that only the individuals withthe qualities of a kingare born with this Haki.
This abilityemanates a shockwave using the user’s willpower, rendering almost everyone in the vicinity unconscious. The only people unaffected by this attack are people with strong willpower of their own, or other Conqueror’s Haki users.
Select few have been able to infuse their attacks with Conqueror’s Haki, making their attacks even stronger, without the need to come in contact with the person on the receiving end of the attack.