The Lightbox (also know as onion skinning) displays the drawings on keys before and after the current key. These drawings are semi-transparent to help distinguish them from the current key.
The display of Lightbox keys before and after can be toggled independently through the main menu (Blue Pencil->Lightbox) and the number of keys displayed can be changed in the settings dialog (Blue Pencil->Lightbox->Settings...).
From the Toolbox UI:
|Toggles the display of keys prior to the current time. Right-click on this button to change the lightbox settings.|
|Toggles the display of keys after to the current time. Right-click on this button to change the lightbox settings.|
Note: For the best performance during playback in Maya, turn off the Lightbox.