The Master List is where all your open tasks and projects live. 

From there you can schedule them into your days or access them through Smart Lists (custom views) to work them off.

One key function of Marvin (or any task manager) is to store all your open items so you don't have to remember them and can manipulate them easily by having a visual representation of them.

Since Marvin also functions as a planner and calendar of sorts, the task storage place is called the Master List. 

If you have used another to-do app before, they were most likely just Master Lists that allows you to keep lists of your tasks and look at those lists through different filters. 

In Marvin the Master List is a separate overlay. Most people benefit from not constantly having to look at ALL their open tasks and projects. We can only work on a fraction of it at any one time anyway.

Setting up the Master List is one of the first things you want to do when starting to work with Marvin.