Triglycerides are fats (lipids) that travel in the blood and provide energy to your body, especially your muscles.  They can come directly from the foods you eat.  Your body processes and stores triglycerides to use later as a source of energy.
Although triglycerides do an important job of providing energy to your body, they can cause problems if they consistently rise too high.  Triglycerides do not directly cause artery blockages (plaque); however, if you have more triglycerides than your body needs, they can be converted to particles that do build up in arteries.  High triglycerides are often a sign of other medical problems, such as pre-diabetes, thyroid problems or other hormonal changes.  Certain unhealthy behaviors also raise triglycerides:  cigarette smoking, lack of exercise, alcohol use, use of certain medications or supplements including steroids.  Sometimes people are born with genetic causes of high triglycerides.
Lab readings usually state that triglycerides above 150mg/dL as abnormal.  However, lipids (cholesterol) specialists now understand that only triglyceride levels under 100 mg/dL are truly normal; therefore we start watching patients more closely anytime we notice triglycerides above 100mg/dL.  
When triglycerides are very high (greater than 500mg/dL) another type of problem can occur with higher frequency:  pancreatitis.  Pancreatitis is an inflammation of the pancreas, the organ where your body makes insulin and substances that help digest your food.  Pancreatitis is very painful and can be deadly, so very high triglycerides can be a serious problem.  Guidelines state that people with very high triglycerides should consider seeing a doctor who specializes in lipid problems (lipidologist) to help them manage their condition.