How to clear textarea on click?

Your JavaScript:

function clearContents(element) {
  element.value = '';
}

And your HTML:

<textarea onfocus="clearContents(this);">Please describe why</textarea>

I assume you'll want to make this a little more robust, so as to not wipe user input when focusing a second time. Here are five related discussions & articles.

And here's the (much better) idea that David Dorward refers to in comments above:

<label for="explanation">Please describe why</label>
<textarea name="explanation" id="explanation"></textarea>

You can use the placeholder attribute introduced in HTML5:

<textarea placeholder="Please describe why"></textarea>

This will place your filler text in grey that will disappear automatically when a user clicks on the text field. Additionally, it will reappear if it loses focus and the text area is blank.