A geyser is a spring that dramatically spews hot water and steam — Old Faithful in Yellowstone National Park is one of the most famous.
When a geyser explodes, hot water and steam shoot up out of the ground like some kind of crazy fountain. It's so amazing that people often use this word to describe things that remind them of geysers. If you suddenly make a lot of money, you hit a geyser. If someone is crying uncontrollably, that's a geyser of emotion. If you're writing quickly and well, it feels like a geyser of words. Anything powerfully flowing could be considered a geyser.