What is significant about Shakespeare's "Macbeth"? Does the play have something important to say about human relationships?
Well - where do you start! Macbeth asks lots of huge, huge questions, which I haven't got space to outline in depth. So I'll give you the brief versions...
1) Freewill versus fate. Who is controlling events? Do the witches actually manipulate Macbeth towards Duncan's murder using magic (we know they cast some sort of spell when they say "Peace! The charm's wound up") - or do they simply push Macbeth a little further down a road which he was already on all by himself? How are we to read the "air-drawn dagger" which appears before Macbeth - or Banquo's ghost? Do the witches actually control or influence Macbeth? Does man really have free will?
2) Power. Many recent productions draw comparisons between Macbeth and Stalin (or Nixon) - and it is true that Macbeth's ruthlessness when he gains the throne is reminiscient of historical rises to power, particularly by tyrants and fascist dictators. Look at the measures he takes - before and after. Look at the importance of an heir (or, rather, the lack of an heir!).
3) The dynamics of a marriage. This is probably what you mean about "human relationships". Lady Macbeth longs for her husband to be king and Thane of Cawdor: and the two seem, initially, to have a hugely close, loving relationship. The play charts her breakdown into madness as their relationship falls apart.
And these three are only the first of many - hope it helps!