I need an explanation of how low blood pressure affects urine formation?

The kidney is a filter. It filters blood to remove excess fluid and eliminate waste products. When the blood pressure is low, there is less blood flowing through the filters and hence less urine formation that results from those individual filters. Each filter in the kidney is called a glomerulus. Normally there are about 3 million filters in the two kidneys. Hence, when we measure kidney function, we measure "glomerular filtration rate" which is abbreviated as GFR.