Optimus is a technology from nVidia that allows you to switch between the lower power Intel graphics that are integrated in your CPU and the higher power nVidia graphics from your discrete graphics card. The big difference between Optimus and other strategies is that even when using the discrete card to do the computations, it still pipes the actual display data through the integrated Intel graphics to your monitor.
There's no set rule for which card to use for what program. In general, you should run demanding applications such as games, modelling software, and GPU optimized programs (e.g. bitcoin miners) using the nVidia Card. The only advantage of using the Intel graphics is that it consumes less power. So first try running it using the Intel graphics; if your performance isn't good enough, restart the program using nVidia graphics. See also the help article here.