A fillet is a thin surface that connects two adjacent surface in order to give a smooth tangent transition between the two. Every single edge on this model has a thin fillet on it. It just makes the model more real to life, so if you render up close, the edges will be nice and smooth, rather than razor sharp. The edge of the wheel well is a perfect example. Look at the way the highlight is nice and rounded/smooth, well that comes from the fillet.