You can't ask this Question if you don't want Manga spoilers, since we've been explained quite a few things about it already (even then its still confusing, but…)
yeah, Almost nobody on this forum will be able to respond to this without telling you what they know about the manga so far.
Its become the Biggest thing to hit the fanbase since "Nakama", and despite almost nothing is known about it, but that doesn't stop most people from claiming it's some kind of magical power.
! It's not Spirit, whatever it really does, it's pretty obvious by now it's the character taking the strength of their Ambition and using it to augment their attacks and weapons or stare someone into submission.
Pay attention to what Hachi says in the newest ep, he calls it "Ambition" (unless some Fansub group left it untranslated completely without a translation note)
Now, beyond that it's a lot of speculation. Some people like me think that's a pretty good explanation right there. Some people think that somehow that ambition is not only some kind of will the character uses to augment their attacks and weapons, but it's pretty much the Force from Star Wars, and Literally explains EVERYTHING IN THE ONE PIECE WORLD!! Including Mantra, CP9's Six Forms (Rokushiki), and How Luffy was able to hit Crocodile and talk to the Millennium Dragons in the Apis Arc.
It all just depends on whether you think Oda will go the Generic Manga route and give them what's basically Chakra, or if he'll take the same route he took with Gear 2nd and took a completely new spin on a shounen staple.