Pyometra literally stands for puss in the uterus. It happens when people don't get their dogs spayed, and I believe it is more common in dogs that have had a litter and then weren't spayed but I might be wrong. It requires very serious antibiotics because the uterus is filled with infection. She will likely have to stay after her surgery for a day or two. Where I work it is considered an urgent matter, and I've seem a few pyo spays and all the dogs have recovered well.