This is widely considered the most important add-on to have installed if you are attending raids or heroic dungeons, as it provides warning of important (or deadly) boss fight event mechanics.
To customise how the add-on displays warnings and plays sound alerts, type /dbm or /dbm help in to your chat window box.
In additional to displaying warnings on your own game client, it can also be configured to send textual warnings to /raid chat, or to send /raidwarning chat alerts to important events. This kind of functionality is most frequently used to indicate with a raid warning message, “Pull in 5 seconds”, with a counting down timer until the tank pulls the boss.