What does the expression “pouring from empty to empty” mean?

Doing something useless and wasting time on meaningless conversations is the same as «переливать из пустого в порожнее» (‘perelivat iz pustogo v porozhnee’) or “pouring from empty to empty”.
The word ‘porozhniy’ here translates as ‘empty’ or ‘unfilled’. So, when you hear this phrase, you will immediately understand: we are talking about some meaningless action. In Anton Chekhov’s play ‘Uncle Vanya’, the main character speaks about his relative, not hiding his disappointment: “For 25 years, he has been chewing over other people’s thoughts about realism, naturalism and all sorts of other nonsense; for 25 years, he has been reading and writing about what the smart have long known, but the stupid are not interested in: that means he has been pouring from empty to empty for 25 years. And, at the same time, what conceit! What pretensions!”
An English equivalent would be: “To beat the air.”