Updating html with js

I am not able to understand what an id is,AND what does variable.innerHTML mean?

The id is a definition and an usage in the HTML. The id attribute specifies a unique id for an HTML element(the value must be unique within the HTML document).Also, id attribute is most used to point to a style in a style sheet,and by JavaScript(via the HTML DOM) to manipulate the element with the specific id.
The variable.innerHTML is the way of setting the value of innerHTML lets you easily replace the existing contents of an element with the new content.It is also a DOM property to insert content to a specified id of an element.Hope this info has helped you,thankyou.

Thanks a lot!It did helped me

An Id is like a unique name given to a variable which will be helpful to select an element with a specific id and will help to give any css styling.
Id is used when we have to apply CSS property to one attribute only.
.innerHTML is used to insert content to a specific id of an element.