Haki

Haki is a characters use of their spirit.  They can channel their spirit to use a "Sixth Sense" as it were, or increase their armor or attack capabilities, and in very very rare cases submit their will upon others. 

Here is a link to further understand those Hakis and what they do.
http://onepiece.wikia.com/wiki/Haki

Haki

One Piece: The Tattered Raven Pirates WiredJester