吗 (ma3) is used to indicate a yes/no question (i.e., a question that is answered by 'yes' or 'no'). Basically, take a statement and add 吗 at the end and it becomes a question as to whether that statement is true or false.

For yes/no questions, you can use an alternate structure that repeats the verb along with a negation. This structure makes the 吗 redundant.

Here are some examples of what this means in practice:

她们是中国人。They are Chinese. (statement of fact)

她们是中国人吗？ Are they Chinese? (yes/no question)

她们是不是中国人？ Are they Chinese? (also yes/no question)

Questions that are not yes/no questions typically have a question word (e.g., 谁 (shei2, who), 哪里 (na3li3, where), 什么 (shen2me, what) to act as a placeholder noun. These replace the subject or object in the statement sentence, for example

她是谁？Who is she?

她是我妹妹。She is my younger sister.

This is really complicated and something Duolingo does not teach well, so I would recommend looking into serious grammar books, if you're really interested in learning about the grammar.

What do you want to know? Sentence structure? Asking questions?

i agree -_- Why are there two?

1. Subject (Plural) - They
2. Verb - Are (comes first in English with interrogatives)
3. Adjective - Chinese
4. Nominative - people
5. Interrogative (implies a request for knowledge).