Cholesterol is a type of fat found in all of your body's cells. Your body needs cholesterol to work properly. In order to get to all of your cells, cholesterol needs to travel through the bloodstream. Proteins form a shell around the cholesterol, making a "cholesterol complex." It is then released into the bloodstream and travels to where it needs to go.
A protein that is linked to cholesterol is called a "lipoprotein." You have probably heard about "good cholesterol" and "bad cholesterol."
The cholesterol complex or lipoprotein that can be bad is called "LDL". LDL stands for low-density lipoprotein. Normally, LDLs transport cholesterol from your liver and deliver it to the tissues that need it. But, if you have a lot of LDLs left over after all of your tissues have been taken care of, the LDLs will "let go" of the extra cholesterol while traveling through your blood.
The good lipoprotein, which is also the "good cholesterol" is called "HDL". HDL stands for high-density lipoprotein. HDL is "good", because it picks up the extra cholesterol that was dropped off by the LDL's.
This is why it is good to have high levels of HDL in your system, and low levels of LDL. Think of "H" for "high" to help you remember this about HDL. And "L" stands for low, which is a way to remember that you want low levels of LDL.