Widespread means far-reaching. For years, email was used only by computer programmers and technophiles. It was hard to imagine, back them, how widespread the phenomenon of digital communication would become.
An idea that's widespread, is believed by many people, even if it's not true. A person such as Oprah, or Gandhi, or Michael Jackson, who has widespread popularity, is loved by many far and wide. A widespread disease is one that many people have––the flu epidemic of 1919 was deadly and widespread, killing between 50 and 100 million people in every corner of the world.