What does the mode parameter do in the webpage.config.js file?

I am working on a project using Webpack, and I happened to notice a parameter in the object called mode.

According to the documentation, it has two possible values (both are a string). One is development, and the other is production.

Below is my webpack.config.js file.

module.exports = {
  // ...
  mode: "development",
  // ...
};

I can already infer that development will be slower, and production will be faster. However, what makes the code slower in development?