Once you have logged in successfully, your Dashboard will be displayed.
Your Dashboard allows you to add or edit content for your website. Changes you make here will then reflect on your website immediately.
The top menu is called the Toolbar and shows you who you are logged in as (top right) as well as some icons on the left showing how many comments need attending to, etc.
On the left you will see the Main Navigation with various options depending on what is installed on your website. Standard options include Posts, Media, Pages, Comments.
On the right you will see the Work Area, when you click on something in the Main Navigation, the content for that section will load in the Work Area.
Click on the various Main Navigation items on your website and see what loads in the Work Area to get a feel for the different sections.