nVidia Optimus is a "hybrid" graphics system. Your laptop will have an integrated low power Intel/whatever GPU and another nVidia GPU as well. Optimus will turn on the nVidia GPU when you started doing anything which required better graphics performance and turn it back off again when you don't need it.

The problem is that without driver support, the extra nVidia GPU is completely useless as the OS won't be able to turn it on. You're forever stuck with the integrated GPU. As you may have guessed, nVidia only support Optimus on their Windows drivers and don't support it at all in any of their other OS drivers. nVidia only give you a massive binary blob as well for the Linux driver so its not particularly great.