Cholesterol travels in the blood attached to proteins called lipoproteins. LDL (low-density lipoprotein) carries cholesterol into the artery walls, where excess can form fatty plaques that narrow vessels and raise the risk of heart attack and stroke, so it is called 'bad' cholesterol. HDL (high-density lipoprotein) carries cholesterol away from the arteries back to the liver, so higher levels are generally protective and it is called 'good' cholesterol.
Triglycerides are a separate blood fat that also raises risk when high. The overall picture, not a single number, determines your risk.
The goal of treatment is usually to lower LDL while keeping HDL healthy through diet, exercise and, when needed, medication. Your doctor can explain which of your numbers needs attention and set realistic targets.