Command is where you set the settings for things you are in charge of. The options available here change quite a bit based on your current responsibilities on the realm.
In all cases, links for actions that contain periods after them, (e.g. Manage Estate...) will lead to a second page where you will be presented with various options as regards your selection (in this example, the second page would list the various classes you can change into). Links without periods after them (e.g. Army Information) will perform their function immediately. Effectively, that means that actions with periods after them can be canceled after initially selecting them, while actions without periods cannot.
This is currently continental weather information notices and any major harvest events. You cannot take any action here, but it can be good to know.
Things you can do
This area is broken down into various groupings based on what your current status to the realm is. As you are given more rights and responsibilities, more groupings of options will appear here.
If you have been given the duties of an army Marshal, there are army specific options the appear here.
Region Lord Commands
If you are fortunate enough to be a region lord, there are quite a few options available to you.
If you are elected or appointed to the position of banker you will have a few powers at your disposal. You control the distribution of food and taxes.
If you are elected or appointed to the position of Judge you will have a few powers at your disposal. You control the laws of a realm and how prisoners are taken care of.
Probably, as the initial editor has never seen them, there are groups, realm, church/religion, trade, etc.