How to achive this simple effect?

Use a solid layer of the color of your choice then add a mask and modify its feather:

The effects which comes from photography is called vignetting or vignnete

another way is to add a spot light and animate that but its harder hope it works

In your comp go to layer new solid  choose make comp size and color

then select the solid layer in your timeline (If the solid layer is not selected instead of a layer mask it will create a shape layer which is not what you need) and choose the elipse tool (or press q until the sllipe tool is chosen) and in the tool bar and click and drag an oval of the desired size then open the mask properties (or press M twice) and modify the  mak feather

This article gives information about several ways to make vignettes.