Blood pressure is the pressure against blood vessel walls, affected by factors like blood volume, stress, vessel condition, and heart health. Normal blood pressure is below 120/80 mmHg, prehypertension is 120-139/80-89 mmHg, and hypertension over 140/90 mmHg. High blood pressure usually has no symptoms but can lead to strokes, heart disease, and kidney disease if untreated through lifestyle changes like diet, exercise, weight loss, not smoking, and medication if needed.