This is part of a series of examples that describe the basic operation of the D3.js force layout. Eventually they may end up in a blog post that wraps everything together. If you missed the beginning of the series, here's a link to first example.
The code in the previous examples has mentioned a property that D3 calls gravity. The force layout uses this property to keep nodes from wandering off the edges of the visualization, something they might otherwise do to avoid overlap.