Omega-3s are polyunsaturated fatty acids. They play important roles in many bodily processes including reducing inflammation and supporting heart health and brain function. They are considered essential fatty acids because they are necessary for health, but cannot be made by our bodies.
The 3 most important types of omega-3s are ALA (short-chained fatty acid), EPA and DHA (long-chain fatty acids). ALA is abundant in plant foods (such as greens, flaxseed, hemp seeds, walnuts and chia seeds), but must be converted to EPA and DHA to be used as anything other than energy by the body. The conversion process is inefficient in humans so we often need other sources of EPA and DHA. The most efficient sources of EPA and DHA are marine based. Fish are high in EPA and DHA, but also contain cholesterol, saturated fat and often toxins. Algae, which is where fish get their EPA and DHA in the first place, is a great pollutant-free option. If you are eating predominantly plant-based to reverse your cardiometabolic disease, including omission of fish, consider eating ALA rich plant foods and obtaining DHA/EPA through a daily algae-based supplement.
You can learn more about omega-3 fatty acids here.