Probably Tarkovsky's greatest film. It's is characteristic of his work in having long takes with visual and sound elements like flowing water, cracking twigs in the forest, etc. mixed in. It contains a wide range of emotions conveyed in contexts that are believable and true to life experience; from treachery to adoration, violence to love. It's the kind of film that can be watched over and over, each time getting different things out of it. It is not a formulaic work conveying a story in a methodical manner, but is more of an 'art' film showing a range of human experience woven around the Russia's famed icon painter Andrei Rubelev in what could be called a period of 'spiritual crisis' in his life.
To enjoy this film, one has to be patient with the long takes and abstract nature elements and not be concerned too much with story and plot continuity, but rather just allow the visual and contextual experience. I think many would include this among the greatest films ever made.