Describe what a ref is at the top of the doc

content/docs/refs-and-the-dom.md

@@ -11,6 +11,8 @@ redirect_from:
 permalink: docs/refs-and-the-dom.html
 ---
 
+Refs provide a way to access DOM nodes and React elements created by React by getting a _reference_ to the element in the render method.
+
 In the typical React dataflow, [props](/docs/components-and-props.html) are the only way that parent components interact with their children. To modify a child, you re-render it with new props. However, there are a few cases where you need to imperatively modify a child outside of the typical dataflow. The child to be modified could be an instance of a React component, or it could be a DOM element. For both of these cases, React provides an escape hatch.
 
 ### When to Use Refs