A mixin is a class whose methods are intended to be added to (or “mixed in” to) other classes.
AnimalMixin is a function that takes a base class and returns a new class that extends the base class and adds the eats method.
Then Employee extends AnimalMixin(Person) - effectively combining Person and Animal.
Also, note that super refers to the parent class, so calling super.description() in Employee will call description on Person, not Animal.

Example:

Person.js

class Person {
    constructor(name, age) {
        this.name = name;
        this.age = age;
    }
 
    description() {
        return `A person named ${this.name} who is ${this.age} years old`;
    }
}
 
const AnimalMixin = Base => class extends Base {
    eats() {
        return `${this.name} eats.`;
    }
};
 
class Employee extends AnimalMixin(Person) {
    constructor(name, age, salary, jobTitle) {
        super(name, age);
        this.salary = salary;
        this.jobTitle = jobTitle;
    }
 
    description() {
        return `An employee is also ${super.description()}`;
    }
}
 
let employee = new Employee("Aurel", 30, 15000, "CEO boss");
console.log(employee.eats()); // Aurel eats.
console.log(employee.description()); // An employee is also A person named Aurel who is 30 years old