Is it okay to ask on english.stackexchange.com questions about meaning of some sentences. For example, I have a quote from Lovecraft and I don't understand what does it mean at all, can I ask it here, or it's better to ask on ELL?
It all depends
For the most part, questions that do nothing but present a passage of text and ask what that passage means are offtopic on both sites. Stack Exchange does not provide paraphrasing services.
These questions are normally too wide open and unspecific to help anyone but the asker, which runs against our mission of building of a library of useful answers that will help future visitors.
These usually show no research, which will get them downvoted, and are generally considered too broad for our format. Interpretation of literature is also offtopic in a general sense, although specific questions about the underlying mechanics of the language used are not.
What you can do is make these questions interesting in the hope that they will attract expert answers. You should be very specific about what you do not understand and why you do not understand it. You should tell us what you have already looked up and where, and tell us why what it said did not help you.
As for which site you place your revised question on, that would depend in part on what sort of answer you are hoping to elicit. If you are looking for answers from linguists, etymologists, and serious English-language enthusiasts, then you should post it to ELU. But if you are looking for answers for people who are just getting started at learning English as a foreign language, then you should post it to ELL.
If it takes merely a native speaker to answer, you probably should post it to ELL. If it requires an expert in English linguistics, then probably you should post it to ELU.
Examples of good “meaning” questions
For excellent examples of how to construct questions about meaning that are extremely well-received here, look no farther than this list of questions from our moderator, Yoichi Oishi ♦.