Translation:You ask the teacher.
My guess would be the context and the implication somebody puts on it. Like in English "You ask the teacher" could be a statement or a command, but by adding "I want you to ask the teacher" or saying it with more force can make it a command. That's just my guess though, there might be a word or character that suggests it's a command.
你们都问 = you all ask (all of you ask)
你们问 = you ask (here, "you" is plural)
你 is the second person singular pronoun in Chinese, equivalent to English "you," used to address one person, "one you," "you and you alone."
你们 is the second person plural pronoun in Chinese, equivalent to English "you," used to address two or more people, that is, "more than one you," or "two or more of you."
Note that in English, "you" is not only the second person singular pronoun, but also the second person plural pronoun: the English translation of both 你 and 你们 is "you." (That is standard English, both standard American English and standard British English. There are nonstandard regional variants, such as "y'all," "ye," "yins," "youse," and "you'uns," among others, which are perfectly legitimate [as variants, but not as the standard], especially where such pronouns are accepted, understood, or even expected.)
都 means "all." Thus, 你们都 means "you all" or "all of you."
The difference between 你们 and 你们都 is that 你们 refers to "two or more of you," whereas 你们都 refers to "all of you (people)." Let's say a boss is addressing a group of workers. There are 20 workers altogether; 5 of the workers have volunteered for overtime; one of the workers won the "Employee of the Month" award. The boss might tell all of the workers, 你们都, to take the rest of the day off, but tell just the 5 overtime volunteers, 你们, to come in two hours early tomorrow, and finally congratulate the 1 Employee of the Month winner, 你. To review:
你 = one you (only one)
你们 = two or more of you
你们都 = all of you, every one of you (but there are at least 2).
In the DuoLingo exercises you have seen so far, Mandarin uses 都 (all) when it already uses 们 (the plural marker), because, "all" refers to "more than one" (plural) in the examples you have seen so far. In other words, why would Mandarin not use 们 with 都? 们 without 都 makes sense, as 你们 is simply "you (in the plural)," but if you use 都, then it makes sense to use 们; because, if you are referring to "all," are you not concomitantly referring to a plural? Without 们, by itself, 你都 would mean something like "all of your person," as if, "all of your being," or "all of your body," or something along those lines: I do not really know what it would mean, but 你们都 makes sense as referring to "all of you people."