The TM side-by-side editor window in the Translation Memories view is where you perform maintenance on your translation memories. It is called the TM side-by-side editor window because the both the source and target segments of each translation unit are displayed in the window, side by side.
The topics in the TM Side-by-Side Editor Window Components book describe the components of the TM side-by-side editor window.
The topics in the Creating and Managing Translation Memories book describe how to use the available functionality when performing maintenance on a translation memory.