A lap desk is a small personal writing surface, consisting of a wood or plastic top over some manner of small cushion. Frequently, these are seen with beanbags to sit between the user and the desk surface, in order to provide stability.
Lap desks are excellent for writing while sitting up in a chair, and make excellent buffers to prevent a laptop computer from getting overly hot or making your legs uncomfortably warm during extended noding sessions. Additionally, some lap desks provide straps to hold down the corners of a book, making it easy to read without having to hold the book open yourself.
Many of the advantages of a lap desk can be obtained by using a simple clipboard, but the extra cushioning present on a lap desk makes the much more comfortable for long term usage, and especially to keep you from frying your naughty bits with an overly-warm laptop.