Systolic is the pressure in the blood vessels while the heart is contracting. Diastolic is the pressure in the blood vessels while the heart rests between beats. The difference between these two numbers is called pulse pressure. For example, if a patient’s blood pressure is 110/70, then that patient’s pulse pressure would be 40. When these two numbers are close together, say 120/110, the pulse pressure would be 10, and would be said to be a narrow pulse pressure.
What that means is that the blood isn't being pushed very far each time the heart contracts. Consequently, the heart will need to contract more often to circulate the same amount of blood. This leads to the heart being overworked, and it can result in a condition called cardiomyopathy. Common symptoms are shortness of breath, rapid pulse, and fatigue. Cardiomyopathy is very serious and can lead to death.
All that being said, one abnormal blood pressure reading means nothing. Blood pressure fluctuates constantly; everything we do effects it. How much caffeine we've had, if we're happy or sad, how much sleep we've had, how recently we've been to the bathroom, EVERYTHING. If a patient takes five blood pressure readings throughout the day, they'll all be different. So until a patient has a clear pattern of abnormal readings, there's nothing to worry about. Of course, the exception to this would be extremely abnormal readings.
As always, anyone concerned about their health should not replace quality medical care with internet advice. If concerned, please see a doctor.