Satoshi Nakamoto is the creator of bitcoin. It may be a single person, it may be a group, but who this is exactly is still unknown and up for debate.
There are many individuals, and groups, who claim to be Satoshi Nakamoto, but no one has yet to provide irrefutable proof.
For someone to prove they are Satoshi Nakamoto, they would need to sign a message with one of the keys associated with the earliest blocks, or move some coins associated with one of his/her/their wallets. Until this is done, be skeptical of anyone claiming to be Satoshi Nakamoto.