While Devil Fruits are known as the predominant power system in mangaka Eiichiro Oda’s One Piece manga series, there exists a more inherent supplementary ability. Known as Haki, it’s a mysterious power that allows individuals in the series’ world to harness their spiritual energy to perform superhuman feats.
There are three types of Haki, two of which can be used by anyone. However, very few people ever learn how to activate either of the two forms, and even fewer achieve total mastery of them. The two universal forms are Armament Haki and Observation Haki, also known as Busoshoku no Haki and Kenbunshoku no Haki, respectively.
Observation Haki allows the user to sense the presence, strength, and emotions of others regardless of distance or sightlines. The range of the ability is instead limited by the user’s skill. Observation Haki also allows precognition via sensing opponents’ intentions and predicting their actions accordingly. Its most skilled users can even see into the future for a brief window of time.

Armament Haki allows the user to turn their willpower and aura into armor, which can enhance defensive and offensive capabilities. Armament Haki can also bypass the intangibility of Logia-type users’ bodies in One Piece and can be applied to one's own body or weapons. Advanced Armament Haki allows users to emit short-range blasts of it without a medium and can even be directed to destroy a target from the inside out.
More often than not, people have an affinity to one of the three types of Haki, with the affinity coming down to one of the two above for most. Typically speaking, one would focus on training and fighting with the kind of Haki they’re best suited to naturally. That being said, one can improve their abilities in all types of Haki, but some may take more work than others to advance.
One Piece’s third Haki type and why it’s the most important

The third and final type of Haki is also uniquely different from the two mentioned above. Known as Haoshoku no Haki, or Conqueror’s Haki, it’s a rare ability that only one in several million people is capable of using. It’s also the most important one, being shown as the most powerful and versatile and as the example of how Haki “can transcend all,” as the series puts it via Kaido.
Conqueror’s Haki, in its base form in One Piece, allows the user to unleash their willpower in a long-range blast or constant emittance, which then overpowers the will of others. Those with especially weak wills tend to fall unconscious, while others are simply mentally overwhelmed to the point of having physical symptoms.
Conqueror’s Haki can also be used to apply pressure to and damage objects, with clashes between Conqueror’s Haki users being potent enough to split the skies above. Like with the other two forms of Haki, there is also Advanced Conqueror’s Haki. Advancement lets users infuse their Conqueror’s Haki into weapons and physical attackings, increasing their power.
Advancement has also been shown to be capable of allowing its user to manipulate their aura or presence. This would then negate an opponent’s ability to sense the user with Observation Haki. While this is considered canon to the series, its only source per the series’ unofficial Wiki is a volume corresponding to the events of One Piece Film: Red.
In summation

In One Piece, Haki generally refers to three unique and distinct abilities: Armament Haki, Observation Haki, and Conqueror’s Haki. The first two are usable and trainable by anyone in the series’ world, while Conqueror’s Haki is a rare gift only one in several million people can use. All forms of Haki can be strengthened via training and have their own unique advancement levels and abilities.