It's a function that returns a function. An example of where you might use this would be to reduce repetition when you need a set of similar callback functions. Here's an example from a recent React Native app that I've worked on: Consider a pocket calculator app that has a grid of similar buttons, each with an onPress callback that needs a distinct value to be used in the callback function:

// Snippet from React component
...
const onPress = value => () => {
  console.log('You pressed ' + value);
}

return (
  <>
    <Button title="ONE" onPress={onPress(1)} />
    <Button title="TWO" onPress={onPress(2)} />
    <Button title="THREE" onPress={onPress(3)} />
  </>
)

Each button gets an onPress callback function with a ()=>{} signature, as required, that does something with a distinct value. Rather than having to declare three functions, we've just declared one. Nice and tidy.