Stands for “fear, uncertainty and doubt”. It’s actually a very relevant term. In a lot of contexts. It’s when people influence you to either fear, have uncertainty about, or doubt something. Sometimes this happens without any intent, for example if you’re unsure about some new situation or thing, you tend to first view it more negatively because you have some fear or doubts about it (which might not be warranted). But sometimes, it can have malicious intent (e.g. right-wing extremists spreading FUD about human subgroups they don’t like. Usually the first step before more aggressive behavior happens).