Summarize the following:
This is important for ensuring adequate air circulation.  The image shows use of wood to make feet; this is a great way to use up scrap wood. Alternatively, small rubber, plastic or similar feet can be purchased from the hardware store. Non-slip designs work best.

Summary:
If your laptop does not have 'feet' under it (pads that raise it up), glue on something to help raise the pad and laptop a bit.