Kidney stones can be a real pain in the, well- Kidneys! Renal calculi (a.k.a. kidney stones) are typically lodged in one of the ureters (tube connecting kidney to bladder). This can block the flow of urine and be very painful. For some people they experience no pain but the stones can become quite large and must be dealt with as if they continue to grow, they could cause a blockage and become painful down the road. If the blockage becomes really bad the urine backs up in the kidneys and can cause serious kidney damage.
Stones are a sign that the body is not in mineral balance. Since the stones are made mostly of Calcium, this tells a bit about what is happening in the body.