A coding standard is a document that specifies HOW a developer should write code. It covers details such as whether to use tabs or spaces, how to structure your parenthesis and curly-braces, etc.
Browsing through a couple coding standards is the best way to learn what a coding standard is.
Also, check out the PEAR coding standard for an example of one written for PHP:
Keep in mind that there is no ONE TRUE coding standard for ANY language. Every developer has their own "preferred" way of writing code. A coding standard enforces all developers to stick to a concrete set of rules and conventions, regardless of what their personal preferences are. The result is that ALL the code written in an application looks the same, which improves overall readability and comprehension.