In general, we accept questions about all aspects of sci-fi and fantasy works, including those that aren't directly related to the sci-fi or fantasy elements. See How should we handle questions that are about non-SF/F elements in a SF/F work? from nearly three years ago.
Questions about the meaning of phrases in a work of sci-fi or fantasy might also be on-topic at one or both of the English-language SE sites, but on-topicness for other sites is entirely irrelevant to on-topicness here. This is something that's been discussed ad nauseam on this and many other meta sites - see for example the above-linked SFF meta discussion, as well as others such as this and this. We shouldn't push questions away to other sites if they're on-topic here. We might recommend people that other sites might be able to answer their questions better, but we certainly shouldn't be telling them not to ask their questions here if they're on-topic.
The only question then is: are such questions on-topic here? To which I say again, why not? They can increase people's understanding/appreciation of a work of sci-fi or fantasy just as much as plot questions can. If you don't understand why a character in an SF/F work did something, or if you don't understand what some sentence in an SF/F work means, then that will hamper your comprehension of the work as a whole, and a site devoted to SF/F works can help you with that.
Not to mention, sometimes answering such questions does require knowledge of the work, just as answering plot question does. See for example What is "paying the iron price"? or What does this passage in A Feast for Crows mean? or What does "master of death" mean? For a less-than-fluent English speaker, it might be hard to tell whether some phrase is something specific to a particular work of SF/F or just a common English saying. The former requires the expertise of our site; the latter could be answered on ELL (but is still on-topic here, IMO). But until you know the answer, it might not be clear which is which - and answers don't make questions off-topic.