SharePoint Add-In is an independent component which is used to include a new functionality or extend the functionality in 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. It is the launching place/web for the SharePoint Add-In.
Add-In Web
A special child web site is created during the installation of the Add-In and all the components mentioned in the SharePoint Add-In are deployed to this web which is called Add-In web.