Upload Center is a tool that is included in Microsoft Office 2010. When you install Office 2010, the Microsoft Office Upload Center installs automatically.

Upload center gives you a way to see the state of files you’re uploading to a server, in one location. When you upload a file to a web server, Microsoft first saves that file locally to the Office Document Cache before it starts the upload, which means that you can save changes and immediately continue working even when you are offline or have a bad network connection.

Microsoft Office Upload Center uses a feature known as Efficient File Transfer (EFT) to help reduce bandwidth usage between SharePoint 2010 and Office 2010 applications.

After installing Office 2010, you may see a small circular yellow icon in your Windows taskbar notification area. This is the Microsoft Office Upload Center icon. Clicking on it will open the Upload Center.

Alternatively, you can also access it via Start Menu > Microsoft Office > Microsoft Office 2010 > Tools > Microsoft Office 2010 Upload Center.

When you open it, you will see the list of all files that have been cached.  You can monitor here, the status of files that are in the process of being uploaded.

You can, if you wish, remove the Microsoft Office Upload Center icon from displaying in the notification area of your Taskbar. You can do so by  un-checking Display icon in notification area check-box in the Display Options in Settings.

You can read more about it at Microsoft.