This was one of the things on the Financial Planner's to-do list for us. My very lay understanding is that it is general liability coverage in case any of your item-specific coverages are insufficient -- say, my auto insurance only covers up to $500,000, and I cause a wreck and am sued for $800,000, then the umbrella policy kicks in.
Or say someone falls down the stairs at my house and sues me for more than whatever limit is in my homeowners' insurance, then the umbrella policy kicks in.
I am not 100% sure that we needed this (so whoever was flaming people for being uninformed consumers yesterday, take note), but it's $11/mo for $1 million in coverage, and the FP recommended it, and I'm willing to risk $11/mo that he's right.
Just random sharing!