Haematuria is the appearance of blood in the urine. Haematuria is not a disease in itself, but it may indicate an abnormality in your urinary system.
Normally, the kidneys are responsible for preventing blood from seeping into your urinary tract and showing up in the urine. Hence haematuria might indicate kidney-related issues such as kidney stones, kidney inflammation (glomerulonephritis), inherited kidney disorders (alport syndrome or sickle cell anaemia) or bacterial kidney infection. Kidney stones are usually common among middle-aged patients and might be a problem if they end up obstructing the urinary tract. Inflammation and kidney infections usually affect the filtering ability of the kidneys, hence allowing blood leakage into the urine.
There could be non-kidney related sources for haematuria as well. These may include an enlarged prostate (resulting in excess pressure on the urinary tract), kidney injury, rigorous physical workout and medications which cause the bladder to bleed.