Hello Vitaly, You can say: What is meant by ------? What does ------(something) mean? What is the meaning of --------a word or something? The error in your phrase "what is mean" : In English you can't use a verb after verb 'to be' which is in the 3rd singular here (is) unless it is in another form other than the infinitive (mean) . The verb (mean) could be in the participle forming the passive voice for example: What is meant by ....... ? - If you don't use verb "to be" you could leave the verb (mean) in the infinitive and say for example: What does ------- mean? Here you are using verb "to do" as an auxiliary verb , it is always followed by the base form (infinitive).

the word 'mean' is not always a verb. For instance, if it is being used as a noun, then your question might be: "What is the mean?"

when mean is a verb,it has a fixed usage that "what do you mean by... /what does it mean?"