Firstly let's take a look at lipids. Lipids are a group of matter which are also found in our body. A more common name used for it is fat. Cholesterol and Triglyceride are two of them.
These lipids are absorbed into our body by eating food. And just like any other thing we dump in our mouth, these lipids join the flow of digestion and after the breakdown, enter the bloodstream. It is known that the blood stream is 90 percent water-based and in obeisance to the law of chemistry, lipids (fats) and water do not mix. So the cholesterol and triglyceride particles can't travel alone in the bloodstream.
So they are carried around in packages called lipoproteins. They are secreted by the liver and eventually return to the liver. Lipoproteins are divided into a couple of groups. The three main ones are:
1) LDL (Low Density Lipoproteins)
2) VLDL (Very Low Density Lipoproteins)
3) HDL (High Density Lipoproteins)
VLDLs carry cholesterol and triglyceride. On their way, they drop some of the triglyceride and become LDLs. So Low Density Lipoproteins carry only cholesterol. LDLs also displace some of their passengers (cholesterol) as they flow along in the bloodstream. That's why these two are called "bad cholesterol". So, they drop some of the passengers off on the way but they themselves and the rest of the passengers are taken back to the liver for metabolism.
On the other hand, HDLs sweep off the cholesterols and triglycerides in the bloodstream. And as high levels of these lipids can cause many problems such as heart problems, HDLs are called "good cholesterol".
So the more HDL exists in our bloodstream, it is better for us but the more LDL rate means trouble, such as atherosclerosis, stroke, heart attack, ischemia, and much more.
There are two ways we can have healthier blood vessels and overall, a healthier heart. One is by eating food with less fat to decrease the amount of LDLs. And two, by eating food which help in increasing the population or effect of HDLs. For example: antioxidants (such as vitamins A, C and E; garlic; onion) and also Niacin (vitamin B3).