This is very similar to the imperative mood.
As “what does the que means”, I don’t really know, I just feel this is just another way to say it, a bit like an idiom, so that’s it.

It means more “Let them in” than “Send them in” (which would be “Faites les entrer”)

This could also have been “Et bien, qu’ils entrent !”.
But then this gives a bit more the feeling that the person saying that is being harassed, in a rush or upset because it’d be something too obvious maybe.

By the way the famous quote from the Queen of Hearts in Alice in Wonderland “Off with their heads” is more often “Qu’on leur coupe la tête !” than “Coupez leur la tête !”.

JeromeJ hits it on the head, if not with clear explanations.

Although French only has Imperative forms in the second person plural and singular, and the plural first person, it is capable of expressing orders and injunctions in other persons (just like English, who only has imperative in the second persons, expressing other injunctions with “let”).

French forms these sentences with the subjunctive preceded by que. According to Grevisse (Le Bon Usage, 14th ed. §407 H2) this is an ancient construction deriving directly from Latin. It used to be possible to drop the que (and indeed various expressions do so: advienne que pourra, soit dit entre nous…), but outside these expressions or derivatives of them this is a high literary way of writing nowadays.

Another very famous line with this “imperative subjunctive” is the Bible’s “Let there be light”, which in French is commonly rendered as que la lumière soit.

It can be an imperative as explained in the previous answers but can also mark the profusion of something. Like in :

« Que d’amours splendides j’ai rêvées ! » – Arthur Rimbaud

→ “I’ve dreamt of so many splendid loves!”

« Que d’eau ! »

→ “So much water!”

« Qu’il est con. »

→ “He’s so dumb.”