Omega 3 is an essential fatty acid, that is, humans need it but their bodies can’t make it. Vegetarian diets are usually high in Omega 6, and it the ratio of Omega 3:6 that is particularly important. The most common way of getting Omega 3 is from oily fish, which is obviously not suitable for a vegetarian diet.
Omega 3 is also found in linseed (flaxseed), chia seeds, walnuts and pumpkin seeds. You can also buy vegetarian Omega 3 oil and supplements. (Most Omega 3 supplements contain fish and krill oil, so take care when choosing. Choosing a plant based product also avoids the issue of heavy metal contamination sometimes found in fish-based products, and ocean depletion from taking food sources from other marine animals such as whales.)
Omega 3 is important for heart, brain and joint health, among other benefits.