A spoiler is information about a book, movie, show, or game that may not yet be commonplace or generally well-known, if knowing it would tarnish the experience for a new audience.
Major plot twists and character developments are usually considered spoilers by most people, but even trivial information can be considered a spoiler by someone who doesn't want to know anything about a work before they experience it firsthand. On the other hand, even a major reveal may become so well-known over time that even someone who has never seen the work can reasonably be expected to know it already. A good example of this is the identity of Luke Skywalker's father. We won't give it away here, just in case, but if you don't know it's [Darth Vader], where have you been the past few decades?
Spoilers can be a problem in a fast-moving fandom after a new release if people do not make note of spoilers in discussions or fanfic. Not warning for spoilers when discussing a new release is considered rude, and spoiling a person or community that has asked not to be spoiled is extremely rude no matter how long the work in question has been available. It also does no good to go "Spoiler alert: spoilerific spoilery spoiler!" without hiding the information or at at least putting in a few line breaks between it and the warning.
Conversely, some people thrive on spoilers, either reading them or giving them. This is okay as long as everyone in the group agrees it is.