What is the difference between Access and Grant in main settings?

Access is short for User Access. This is the place you can give other Pagedip users (or an entire organization) the rights to review or change PageDips you create even prior to them being published.

Grants are a way to link content between PageDips in your library. It's especially useful when you want to share assets from one central location and create a Content Hub of standard components that all your PageDips draw from.