An antagonist is someone who opposes or works against the main character in a story. In The Lion King, Scar is the antagonist because he schemes against Simba and tries to prevent him from becoming king. In Charlotte's Web, the farmer who plans to slaughter Wilbur is the antagonist, even though he's just doing what farmers normally do.

The antagonist creates the conflict that drives the story forward. Without Voldemort opposing Harry Potter, or the White Witch threatening Narnia, those stories would have no tension or excitement. Notice that antagonists aren't always purely evil: sometimes they believe they're doing the right thing, or they have understandable reasons for their actions, even if their methods are wrong.

Outside of stories, you might hear someone described as an antagonist when they consistently oppose or challenge others. If two students are antagonistic toward each other, they're in conflict, perhaps competing for the same position or disagreeing about how something should be done.

The opposite of an antagonist is the protagonist, the main character whose journey we follow. Understanding who the antagonist is helps you understand what obstacles the protagonist must overcome.