In this blog , I will talk about AppWebPart/ClientWebPart in SharePoint 2013. As you may know that , you can develop SharePoint Apps to integrate with any site Page. You can publish your App in Office App Store to enable users to download the app.
The main concept behind SharePoint Apps Development is ClientWebPart. Client App Apart is a concept which let you to run SharePoint pages inside the iFrame. Thus you can display any page inside the client app part.
<WebPartPages:AllowFraming runat="server" />
This tag allows to page to run your SharePoint Page as a Iframe. When you develop SharePoint Apps using NAPA Tools , ClientWebPart Page comes by Default to make the required Changes.