This is a reference to the "new" state syntax introduced in Flex 4.0. It means that when the state of the component is set to disabled the alpha property will be 0.5 . Otherwise the alpha state will be the default value--which I I assume 1.
When creating my own components; and using this approach, I like to specify values for every state. Sometimes I use the 'catch' all. Like this:
<s:SparkButtonSkin xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:fb="http://ns.adobe.com/flashbuilder/2009"
minWidth="21" minHeight="21"
alpha="0.7"
alpha.normal="1"
alpha.disabled="0.5"
>
The above code means that in the disabled state the alpha property will be 0.5. In the normal state the alpha will be 1. In all other states the alpha will be 0.7.
The 'state' name after the property can also refer to a state group instead of an explicit state.