Could ground and center mean to clear a negative conscious and be more positive? I want to do this to have a more positive attitude.

It's the stage before. You center yourself so that you are connected to your own being (rather than all the dross), and ground yourself so you are connected to everything around you. You are essentially bringing yourself in harmony with both aspects, into stillness. Once there, negativity has no place. It can be fed to something externally (a candle to cleanse, the earth, etc.), but generally you need that emotional zen-like calm to consider it without feeling negative about it, and then either resolve it internally or be rid of the build-up externally.