It doesn't "mean" anything particularly. It simply indicates that we are not being precise about the identity of the person or object, usually because we have not mentioned the person or object before.
"She is a lawyer." This is an answer to the question "What does your sister do."
"She is the lawyer." The use of the definite article "the", instead of the indefinite article "a", means that the speaker is sure that the other person knows which lawyer, exactly, is being talked about -- usually because this has already been mentioned in the conversation:
"Initech Corp. has a big legal problem."
"How do you know?"
"My sister is the company lawyer -- she's trying to sort the problem out."
The company has already been mentioned, so "the company lawyer" means "the lawyer of the company which I mentioned before".
"A lawyer" is a lawyer in general. "The lawyer" is a specific lawyer.
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