Runaway just means something is out of control. "The runaway car rolled down the hill and crashed into the school". Runaway debt means the government is spending a lot more money than they are receiving. The level of debt that they are creating is out of control - 'runaway national debt'. Looming is used to describe something big that will happen in the future. "The looming storm will damage many houses". Across the board means something that will apply to everyone or everything. "The company gave an across the board bonus at Christmas". So combining them, 'looming across-the-board' means something big that will happen in the future and apply to everyone or everything. "Take effect absent' is a way of saying something will happen regardless of something else. Even though they do not have a deficit reduction agreement, the spending cuts will happen anyway.

This refers to the fact that America's debt continues to grow more and more everyday and that this debt can't easily brought under control. An example would be like a runaway tractor trailer (a huge truck) traveling down hill without brakes. This is considered a runaway truck. It can't be stopped and it will continue increasing speed as it goes along. It is the same thing for our debt. At this moment it can't be stopped or controlled