An anonymous function is a function without a name.
Converting sayHello() function into an anonymous function:

function sayHello() {
  console.log('Hello');
}
function executeCallback( callback ) {
  callback();
}
executeCallback(sayHello);

Becomes:

function executeCallback( callback ) {
  callback();
}
executeCallback(() => console.log('Hello'));

This is also know as inlining or writing a function in line.
It can be more expressive as a callback function is more obvious when it’s written as an anonymous function.