The crown is the concrete slab on top of your chimney. It sheds rain away from the flue and the brick below. When it cracks, water runs straight into the masonry – and in North Texas the freeze-thaw cycles widen those cracks every winter.
Crown repair resurfaces a crown that is still structurally sound: I remove the loose and flaking material, rebuild the surface with a flexible crown coating, and seal it so water sheds off instead of soaking in. If the crown is broken all the way through, a resurface will not hold – I will tell you that on the roof and quote a chimney crown rebuild instead.