Someone and somebody (the former is used more in written texts, the latter in spoken language) refers to a specific but indefinite person.
It is important to note that nouns in English are marked as being both definite or indefinite and as being specific or non-specific. Someone works much like a/an when a/an refers to a specific, indefinite noun.
1 I have to see someone about my homework tonight.
the identity of the person is known (specific) but not revealed (indefinite)
2 I have to read a book for my homework tonight.
the identity of the book is known (specific) but not revealed (indefinite)
Note that in English a/an is also used to make a non-specific, indefinite reference:
3 I need to by a new car.
If the speaker does not have a specific car in mind, then the use of a is non-specific and indefinite. Which is analagous to any.
The analogous pronoun here is anyone/anybody.
4 Can anybody give me a ride home tonight?
Anybody refers to a non-specific, indefinite person. The speaker does not have a specific person in mind when asking the question.
5 Can somebody/someone give me a ride home tonight?
Someone is a specific, indefinite person. The speaker believes that there is a specific person who can give them a ride home, even if the identity is unknown (indefinite).
Notice an actual example of this on ELU:
6 Can someone explain why this question was downvoted?
The speaker Michael Hardy is referring to a specific but indefinite person. This usage represents confidence on the part of the speaker that there is a person (specific but indefinite) who can explain.
Had the comment been
7 Can anyone explain why this comment was downvoted
the speaker is referring to a non-specific, indefinite person. The speaker is not sure if there is a (=any) person who can explain this.
Now more uses of someone (somebody), in which all of them refer to specific but indefinite person.
8 Someone's coming over to dinner tonight.
9 I saw somebody kissing Aunt Mae.
10 Someone left their book on the table. Perhaps its Peter's.
11 Somebody can just walk in and take anything they want, couldn't they?
12 There's someone at the door who wants to speak to you.
13 I can hear the fire trucks. Somebody's house must be on fire.
14 Can I have a someone over for dinner?
15 Somebody shot the sheriff.
16 Someone's been sleeping in my bed.
17 I really need to get help from someone.
Again, the speaker is making a specific but indefinite reference. The speaker believes a specific person exists who can help them, but their identity is not known (indefinite).
To recap, consider hearing a noise in a dark alley:
When the speaker asks:
18 Is somebody there?
the speaker is referring to a specific but indefinite person; i.e., the speaker believes that there is a specific person who made the noise, but that specific person's identity is unknown (indefinite).
If the speaker asks:
19 Is anybody there?
the speaker is not sure whether a person made the noise and so makes a non-specific, indefinite reference. The speaker is not referring to a specific person.