How can I understand DOM very well & what is exactly DOM ?

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Javascript

I did write a Code with JS trying to understand what is DOM in JS based on some online documentations and tutorials but its really not clear for me since begin a new in JS thus i have wrote a code do some basic staff and need an explanation
if someone could explain to me the DOM based on my code i will be so glad .

What I have tried:

<div id="david">Hello World</div>
<button id="clickMe" onclick="showmessage()">Click Me</button>
<P id ="GetArray"> </p>

<script>

function showmessage () {  
	var GetText = document.getElementById("david") ;
	if (GetText.innerHTML=="Hello World"){
		GetText.innerHTML="Hello FXXL";
	}else {
    GetText.innerHTML="Hello World";
	}
}
function SortArray (value) {

text += "<li>" + value+ "</li>";

}
var text = "<ul>";
var data = [
  "Microsoft",
  "Apple",
  "Google",
];
data.forEach(SortArray);
text += "</ul>";
document.getElementById("GetArray").innerHTML=text;
</script>