Passing execution context in JavaScript from html elements

I am very new to JavaScript programming, and i am trying to do form validation using JavaScript.

While learning form validation i was reading about how to disable submit button until all input fields are validated, i saw many techniques to do this, but, i am confused regarding the following code:

<body>
    <h3>Enter some values in the text to enable the button!</h3>
    <input type="text" id="txt" onkeyup="manage(this)" /> //focus here on "this" parameter
    <input type="submit" id="btSubmit" disabled />
</body>

<script>
    function manage(txt) {
        var bt = document.getElementById('btSubmit');
        if (txt.value != '') {
            bt.disabled = false;
        }
        else {
            bt.disabled = true;
        }
    }    
</script>

Here this is passed as an argument from an input element event listener in html. I tried passing something other than this from there, it didn't work only passing this works,

As far as i know this in JavaScript signifies current-execution-context, but since this is JavaScript keyword then how it is interpreted inside html?

JS FIDDLE LINK while passing 'this': https://jsfiddle.net/s0vnjpqb/

JS FIDDLE LINK while passing 'some': https://jsfiddle.net/s0vnjpqb/

PS: I am new to JavaScript, hence the question is easy, this might not match the standards of stack-overflow, but i tried researching in stack-overflow as well as on other platform i couldn't understand it.