Veins are blood vessels. They are small tubes in the body, sometimes longer, sometimes thicker. The heart pumps all the blood through a person's body through the veins once a minute. Added up, that would be about 7000 litres of blood in a single person, in a whole day. Not only humans, but also most animals have veins.
If you were to string together all the thick and thin veins in a human being, you would arrive at a total length of about 100,000 kilometres. That is two and a half times around the earth. This is called the circulatory system, in which the blood is brought to all parts of the body. All organs such as the eye, liver, lungs, nose or brain receive blood in this way, but also the bones, muscles and skin.
If a vein is blocked or ruptured, you can get sick. Maybe there is a stroke in the brain. If you have a heart attack, your heart stops beating. There can be even more illnesses if the blood circulation is disturbed. That's why the doctor looks to see if the blood is flowing through the veins undisturbed. In addition, the walls of the veins should be stable.
The system of veins forms the blood circulation. It is more or less the same in humans and birds. It consists of four parts, which can be imagined like tree trunks with branches and ever finer twigs. Two trees form the arteries, two trees form the veins.