'Someone' refers to a person without saying exactly who they are. 'Someone' is more formal than 'somebody'. It refers to a person who is not known or mentioned by name.

'Some one' refers to one unspecific person in a group of things or people that we are choosing from.

Remember that almost all the time, when we use 'some one' it has to be followed by 'of + noun phrase)'.
When it is not followed by a noun phrase, it's written as 'someone'.