It's an API for building custom charts. It's a huge step up from having to fully 'roll your own'. If you have to do any kind of vis that isn't available in the various off the shelf libraries, you can put it together with D3 easier than you could if you fully DIY a solution. It's well thought out but you have to use it for that to be more obvious. This book (by the D3 author) got me started, it's worth the hassle of learning [0].

[0] http://alignedleft.com/work/d3-book