Union types are a feature in programming languages that allow a variable to hold values of different types. For example, a variable can be defined to accept either a string or a number, providing flexibility in how data is handled. This is particularly useful in scenarios where the type of data may vary, such as user input or API responses. In languages like TypeScript or Haskell, union types are explicitly defined, enabling developers to create more robust and type-safe code. By using union types, programmers can better manage the complexity of their applications and reduce the likelihood of errors related to type mismatches.