How is the Spring MVC spring:bind tag working and what are the meanings of status.expression and status.value?

Let's discuss on the following example:

<spring:bind path="user.userName">
    <input type="text" name="${status.expression}" value="${status.value}"/>
    <span class="fieldError">${status.errorMessage}</span>
</spring:bind>

When this view snippet gets rendered, what do ${status.expression} and ${status.value} get evaluated to? Where do these values come from?