Avinu Malcaynu is a way of using English letters to phonetically spell out what in Hebrew translates to: 'our father, our king'.
As you would probably assume, Jews are referring to God when they sing this song. It has a nice, cryptic melody, as do many Hebrew chants, and is often sung on Pesach (passover).

Now that I have heard Phish cover this cryptic and dramatic hebrew song, I am brimming with even more respect for them.