A fatty acid is a molecule made up of a carboxylic acid with a chain of carbons connected to it. Whether these fats are saturated or unsaturated depends on how many carbons are in the tale of these molecules, and how many bonds hold them together. I want to keep this very practical so we won’t get too scientific.
Basically, high amounts of saturated fats in your diet is unhealthy. Fried greasy foods are high in saturated fats. Unsaturated fats are healthy. You can get these fats through a diet of nuts, fish, olive oil, etc. You can also take flax seed or fish oil supplements which are good sources of unsaturated fats.
The body uses these fatty acids as a powerful source of energy during activity. When the body needs a high amount of energy it metabolizes fatty acids because they produce the most ATP.
Fatty acids are stored in triglycerides which make up fat cells. Triglycerides basically have 3 fatty acid molecules. When the body needs the fatty acids for energy, the triglyceride breaks apart. When the fatty acids are not connected to anything they are considered “free fatty acids” because they freely float around the blood until they are metabolized for energy.