system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 What is blood pressure?

What is blood pressure? How is it taken and what do the numbers mean from your results?
gpt
As the heart contracts (systole) it ejects blood and sends it throughout the arteries to the rest of the body. Then it relaxes (diastole) and refills.
This cardiac cycle puts outward pressure on the walls of the arteries. The pressure (mmHg) is greater when it contracts and lesser when it relaxes. i.e. 120/80 mmHG. 

The typical way it is measured is inflating a cuff on the upper arm until the cuff's pressure is greater than the blood pressure and it mushes the artery closed. Then the cuff is gradually deflated and a stethoscope is used to hear when the artery starts reopening, this is the systolic pressure (top number). Eventually, as the cuff pressure drops the artery stops vibrating the stethoscope. The pressure at which that happens is the diastolic pressure (bottom number).

High blood pressure means the heart is working/pumping harder to get blood to the body. This puts strain on the heart and arteries and can create and/or magnify problems.