If you are newbie for SharePoint apps development, you might be wondering to know clear idea about the terminologies host web and app web (add-in web).
The special site where the app is deployed is called an app web. The site where the app is installed is called the host web.
In other words, the SharePoint add-in which host SharePoint components (e.g. lists, content types etc.) is called the app web.
Consider that you are developing an app with custom ribbon action in List view page and the custom action click redirects you to the special site page. Here, the custom action placed in host web and special page is an app web
App Web - The special website to which the add-in is deployed
Host Web - The site on which the App is installed
For more detail checkout this article : https://msdn.microsoft.com/en-us/library/office/fp179925.aspx