SharePoint Add-In is an independent component which is used to include a new or extend the functionality to the SharePoint. The SharePoint Add-In can access data and use components between two webs called Host web and Add-In web.
Host Web: A SharePoint site used to install the SharePoint Add-In is called Host web. The launching place / web for the SharePoint Add-In.
Add-In Web: A special child web site will be created during the installation of Add-In and all the components mentioned in Add-In are deployed to the web is called Add-In web.