Omega-3 fatty acids are essential fats we need for optimum health. Unfortunately, most Americans don't get adequate amounts of omega-3s in their diets, and as a result are more likely to develop cardiovascular disease, cancer, inflammatory disorders, and mental and emotional problems.
The rest of Dr. Weil's answer is here.
What I have to say: In paraphrase of Dr Weil's answer, unless you are eating fish daily or exclusively (think Eskimo diet) you are probably NOT getting enough Omega-3 fatty-acids. I take two to three supplements a day to keep my inflammation under control for my disease. I probably don't eat enough fish or walnuts either, which is something the good doctor recommends.