Android widget update frequency 0, what it actually means?

I have found the following parts of code in a widget:

<appwidget-provider>
    android:updatePeriodMillis="0"
<appwidget-provider/>

Now, my question is now that: what it does mean with updatePeriodMillis set to 0.
I know that it is set for the how often the widget will receive remoteview or others. so, what is the significance to set it to 0.