"These are girls."
Translation:Das sind Mädchen.
That would mean "These ones are girls", i.e. if you were picking out a specific sub-group out of a bigger group of things you had been talking about before.
When you are introducing something new to the conversation by "pointing" at them ("This is..." / "That is ..." / "These are ..." / "Those are ..."), you use neuter singular in German (Das ist... or Dies ist... or Das sind... or Dies sind...), with the neuter singular das/dies being independent of the gender and number of the thing(s) and only the verb agreeing in number.
Ah, I was imprecise. I've edited it now.
The das or dies is always neuter singular, but the verb agrees in number (singular/plural) with the thing(s) being introduced.
So both Das ist [...single thing] and Das sind [...multiple things] are common.
Das sind Mädchen needs to have the third person plural verb sind because Mädchen is plural.
In the singular, it woudl be Das ist ein Mädchen.
Das ist Mädchen is not possible for the same reason that "That is girl" is not possible -- countable nouns generally have to have some sort of determiner in front of them in the singular, such as an indefinite article.
It appears simply, similarly to many English words, to have its function determined by the context of the sentence.
We're just used to different words being determined by different context. ("The man running to the office had a nose that was running and he was running a slight fever, but he was running late for the press meeting about him running for office." Each instance of "running" and both instances of "office" have different meanings according to their context within the sentence, but they're written the same.)
In English, you need to use "this" or "that" when you are introducing a singular thing, but "these" or "those" when you are introducing a plural thing.
For example, "this is a man" but "these are men".
In German, singular dies or das is used in both cases: das ist ein Mann; das sind Männer.
As I understand: "du bist..." = "you are..." (to one person only), making "bist" the second person singular form of "to be". ("you" being second person singular.)
"sie / wir sind..." = "they / we are...", making "sind" the third person plural (sie / they) and first person plural (we) form of "to be". ("Sie" is also used as a formal form of second person singular & plural.)
Basically, in English we would simply say "are" in all the above cases... ("we/they/you are..."), but in German it's conjugated a little more specifically depending on the situation.