What is this drug for and the side effects?

Simvastatin belongs to a group of drugs called HMG CoA reductase inhibitors, or "statins."

Simvastatin is used to lower blood levels of "bad" cholesterol (low-density lipoprotein, or LDL), to increase levels of "good" cholesterol (high-density lipoprotein, or HDL), and to lower triglycerides (a type of fat in the blood).

Simvastatin is also used to lower the risk of stroke, heart attack, and other heart complications in people with diabetes, coronary heart disease, or other risk factors.

Simvastatin is used in adults and children who are at least 10 years old.

https://www.drugs.com/simvastatin.html