Good question, and something I've been trying to make sense out of for a long time. As best I can tell the speed of light is itself relative to the fabric of space. And since space is not matter, it can stretch any way it likes. I picture it as a piece of graph paper. If the horizontal scale is in years and the vertical scale is in light years, then "you can't travel faster than light" means that a graph of your motion can't rise more rapidly than the diagonal of that square. But the squares themselves can stretch, as if the graph were drawn on a rubber sheet.
Keep in mind that this is not my area of expertise, just something I've read about and tried to understand.