There are some guides and posts out there that can explain better than I can in a comment, however, Witnesses are responsible for keeping a reliable, stable server running .. staying up to date with the latest version of the software, keeping up with the community and generally staying active.

Other things such as keeping a steem price feed running. I personally have an autoscaling setup that will be able to handle any load the blockchain may put on it. As my funds increase, I will also run things such as full seed nodes that add into the stability of the network even more.