The top fields will allow you to set the size in pixels and the bottom field will allow you to set the size in an arbitrary length unit. The difference between the two of them is that if you specify the the size in e.g. inches then the size in pixels is dependent of the image resolution. I.e. if you decrease the default resolution, you will decrease the size in pixels, but not the real size.
Let you set the default image resolution and resolution unit. The default image resolution is internally always measured in dpi (ppi) while The GIMP lets you choose the resolution unit you will see in dialogs.
Lets you set what type of image you want to create by default (either RGB or Grayscale).
GIMP will warn you if you try to create images larger than this value.
Does the size & unit contra resolution image/unit sound a bit complicated and hard to understand? Let us give you an example. You are familiar with the metric system, but, like most designers, you are more familiar with a resolution based on pixels per inch (ppi or dpi). Then you have to set image unit to millimeters (or centimeters), while you will set the resolution unit to pixels/inch. This will give you the comfort of working with two familiar units — in this case size measured in the metric system and resolution measured in ppi.
Lets you enter a text which will be attached and saved as comment with many file formats. This can be editted on save.