The blood pressure in the glomerular capillaries pushes plasma through the filter into the urine. The amount of plasma filtered every minute is called the glomerular filtration rate. Your body tries to keep this rate constant at around 125 mL/min.
Because it's so important to keep the filtration rate constant, your body has special mechanisms to control the blood pressure in the glomerulus.
Blood enters the glomerulus through an afferent arteriole. If your kidneys are filtering too fast, you can constrict this arteriole to slow down the blood flow.
If your kidneys are filtering too slowly, you constrict the efferent arteriole that takes blood out of the glomerulus. With blood still coming in quickly, but unable to leave quickly through the constricted efferent arteriole, the pressure in the glomerulus will rise and more fluid will be filtered into the urine.