A prototype in simplest terms is a blueprint of an object. It is used as a fallback for properties and methods if it does exist in the current object. It's the way to share properties and functionality between objects. It's the core concept around JavaScript's Prototypal Inheritance.

  const o = {};
  console.log(o.toString()); // logs [object Object] 

Even though the o.toString method does not exist in the o object it does not throw an error instead returns a string [object Object]. When a property does not exist in the object it looks into its prototype and if it still does not exist it looks into the prototype's prototype and so on until it finds a property with the same in the Prototype Chain. The end of the Prototype Chain is the Object.prototype.

   console.log(o.toString === Object.prototype.toString); // logs true
   // which means we we're looking up the Prototype Chain and it reached 
   // the Object.prototype and used the "toString" method.