The Notification area is a new name for what used to be called the System tray, the area where the clock and various other (active program) icons are displayed.
Not only has the name changed, but the new area provides more meaningful notifications. It also has a mechanism that reduces clutter on the taskbar. Icons that are inactive become hidden. You are informed periodically that icons are inactive, and you can modify this behavior with customization settings, or disable this altogether.
To have more usable area in your taskbar, you may permanent hide some unused icon. To customize the Notification area, right-click the Taskbar and select Properties. Under the Notification area caption on the Taskbar tab you can select to show or hide the clock, and show or hide inactive icons. Select "Always Hide" option for the icons you are not using.