FUD stands for Fear, Uncertainty and Doubt.
It is the spreading of misinformation of a cryptocurrency by uninformed sources.
Many of these sources have their own nefarious reasons for doing so. It may be to promote an alternative coin, or they may have shorted (bet on the price falling) the mentioned cryptocurrency.
Legitimate criticism of a certain cryptocurrency is fair game and should be encouraged to motivate technical improvements.
FUD, on the other hand, is mostly slander and baseless accusations about a cryptocurrency (such as bitcoin) as a technology, the people behind it or the people invested in it.
How can you avoid FUD?
Seek cryptocurrency news from reliable sources, who don’t have a vested interest one way or another. Also, it is important to use multiple sources to get a well-rounded perspective.