It's about a Russian boy whose village is destroyed by the Nazis, and he becomes a prisoner. He manages to escape and ends up convincing the Russian army that he'll spy for them, and he does.
This is a fantastic film, and Tarkovsky's first. Apparently, he's not even credited in this. I can't say much about this film without spoiling it.
It's about the loss of childhood innocence. I knew when I was watching it that if I wanted to say something about it, it would just ruin it.
A lot of directors were influenced by this film (Bergman etc), but Tarkovsky itself wasn't pleased with it. I can understand why to some degree, as there's only small bits that emphasise his style.
Oh, if you've seen Come and See, this is similar, although not as terrifying.