Solution Explorer is a window within the VS.NET editor, which allows you to manage various files part of the solution (web site). This windows displays all the files part of the web site. You may select any file and view properties, rename, delete or move to other folders through the solution explorer.
By default, Solution Explorer is located on the top right corner of the VS.NET editor. This windows may be minimized on the top right bar. If for some reason, you cannot view the solution explorer, go to the menu and select "View > Solution Explorer". This will launch the solution explorer window.
You may click on the tab to expand the solution explorer, as shown below.
The solution explorer shows the list of files associated with the project. You may double click on any file to open it in the editor.
Note the icons in the top of the solution explorer.
These icons allow you to do certain things related to the project. Some of the icons will be active only when an aspx page is open in the editor.
The first icon in the above image allows you to view the source code (code behind file) of an aspx page. The second icon allows you to view the aspx page itself.
See the 5th icon. This allows you to view all files which are in the same directory as the web project but not included in the project. Click on this to view the files in the same folder which are not included in the project. Probably you may not have any files to see. Add some files manually to the project folder and then click on this icon. It will show you the files you copied manually.
You can right click on any file and include in the project or exclude from the project.
Click on the same icon again to hide non-included files.