は is the topic marker. A topic has no grammatical relationship to any words and phrases in a clause. A topic can be like a title of a book or a frame of a picture, just labeling the whole sentence.
The following is an example to show how the topic marker は is used in a conversation:
A: What does Mr. Takana do for a living?
B: He (= Mr. Tanaka) is a student.
The two people are talking about Mr. Tanaka. Mr. Tanaka is the topic, which is mentioned already by the first speaker A in this case, as you see the second speaker (person B) rephrase Mr. Tanaka to the pronoun 'he'.