The macula densa abuts the distal convoluted tubule of the nephrons in the kidneys. It senses salt concentrations (sodium, potassium, and chloride), which are indicative of how much filtrate the kidneys are processing. If the levels of these ions are too low, the macula densa stimulates the juxtaglomerular cells to secrete renin. This will increase blood pressure via the renin-angiotensin-aldosterone system, thus restoring the glomerular filtration rate to its proper level.
These are my interpretation of my lecture notes, but I may have used some references from Hole's Anatomy and Physiology (Shier, Butler, Lewis) and Human Physiology (Vander, Sherman, Luciano)