A hematoma is a collection of blood, sometimes caused by internal bleeding, trauma, etc. A perinephric hematoma is bleeding around a kidney. Signs and symptoms depend on what caused the perinephric hematoma and how bad it is. Signs could be nothing more than microscopic hematuria (blood in the urine) that the patient doesn't notice, but shows up in the lab. It can also be visable bleeding in the urine, abdominal and back pain, nausea and vomiting. If the bleeding is extensive, there may also be shock from loss of blood.
Treatment depends on what is causing the bleeding. If it was trauma, then surgery to repair the abdomen, and the kidneys if they were directly harmed. We see these in the ER from stabbing and gunshot wounds, but they can also come from a motor vehicle accident when the kidney or other internal organ is bruised. If it is just microscopic hematuria, then watchful waiting for the next 24-48 hours to make sure nothing else develops. For more serious cases we give broad-spectrum antibiotics and whatevery other treatment is required.