You have to call the method, which will return a string of the person's name, and then that will be passed to the greetings function.
Without the () the method is not executed and the method itself is passed to the greetings function.
Since it is treated as a string in the function you get the toString() method being called on the function which results in the function code itself.

function greetings(name){
    document.getElementById('someText').innerHTML = 'Hello' + ' ' + name;
}

const person = {
    firstName: "John",
    lastName: "Doe",
    id: 5566,
    fullName: function() {
      return this.firstName + " " + this.lastName;
    }
  };
  

  greetings(person.fullName());
  console.log(person.fullName.toString());

<div id="someText"></div>

you can solve your problem in two ways.

//firstly

 greetings(person.fullName()); // calling the function when you pass it as a Arguments. 

// secondly
function greetings(name){
    document.getElementById('someText').innerHTML = 'Hello' + ' ' + name();
// if you always except a function as a peramiter just call it when you use it.
}

you define fullName as a person object property which is a function. For getting the proper value you have to call the function. we used () after the function name for calling it. if the function didn't call it returns the function body.

since you defined fullname as a function here:

fullName: function() {
  return this.firstName + " " + this.lastName;
}

then you have to call it, like: greetings(person.fullName()); with the additional brackets at the end