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"Someone" can be used to denote the presence of an unknown, or unnamed person:

eg "there is somebody at the door".

It might be helpful to note that the opposite of this definition would be either "nobody" or "no one":

"There is nobody at the door".

The second definition of "someone", which is what your question is about, is "a person of significance", or "someone noteworthy".

eg "One day I'm going to be somebody".

Interestingly, the words "no one" and "nobody" are also used to express that somebody is not significant:

He's a nobody.