Before starting development in SharePoint Add-in, it is really important to understand what is host web and what is add-in web.

Host Web:

When an add-in that includes SharePoint components is installed on a website, it is listed on the Site Contents page from which it can be launched. That listing, which is the launch point of the add-in, is the only required ADDITION to the website, although certain other things can OPTIONALLY be added, such as a CUSTOM action or an add-in part. The website to which the add-in is installed is called the host web.

Add-in Web:

Other than these UI elements, the SharePoint Add-in components and content, such as LISTS, content types, workflows, and pages, are deployed to a different website in a special isolated domain. This fact is largely hidden from the user. The special website to which the add-in is deployed is called an add-in web.

Before starting development in SharePoint Add-in, it is really important to understand what is host web and what is add-in web.

Host Web:

When an add-in that includes SharePoint components is installed on a website, it is listed on the Site Contents page from which it can be launched. That listing, which is the launch point of the add-in, is the only required addition to the website, although certain other things can optionally be added, such as a custom action or an add-in part. The website to which the add-in is installed is called the host web.

Add-in Web:

Other than these UI elements, the SharePoint Add-in components and content, such as lists, content types, workflows, and pages, are deployed to a different website in a special isolated domain. This fact is largely hidden from the user. The special website to which the add-in is deployed is called an add-in web.