Some modern translations call it a bond of indebtedness.
Christ wiped out a note of debt. He canceled our spiritual debts, our sins, our transgressions of God's law, and this is what the note of debt refers to. In his crucifixion, Christ symbolically nailed our note of debt to his cross because his sacrifice paid our debts. That makes the most sense in the context, because our spiritual debts have come from breaking God's laws. However, some commentators have erred in saying that God's laws have been against us and were nailed to the cross.