A placeholder is a textbox that hold a text in lighter shade when there is no value and not focused.
Below is how a placeholder should look like if no value is entered and textbox is not focus. (What you see is just picture, not real placeholder, don't try to click it)
Once the textbox gets focus, the text goes off and you shall input your own text.
Without going further, let's have a simple demo
<label for="first_name">First Name</label> : <input id="first_name" placeholder="First name goes here">
Let's see what which browsers support placeholder.
If placeholder is just a nice-to-have feature in your web page, you probably will not bother about supporting browsers other than above.
<label for="demo">Placeholder demo</label> : <input id ="demo" placeholder="Support Placeholder" />
"testAttribute" function above is to test if an attribute of an element is supported by a web browser. In this case, we want to know if your web browser supports "Placeholder" attribute of "Input" element.
If you are using a web browser that supports "Placeholder", you should be able to see "Support Placeholder" in the textbox. Conversely, you will see "No Placeholder Support" if you are using web browser such as Internet Explorer 8.