Visualising is like deliberately imagining something using all your senses. As your skill develops, your experience will be more vivid.
Grounding and centering refer to your personal energy and the energy of the earth.
First comes centering, where you can focus the energy center of yourself in one place, like your tummy or your womb. You may like to begin with exercises that help you to connect to your personal energy.
With the grounding part, you can use visualisation and will to move the energy from where you have centered, and into the earth. Typically people use the image of a root, or roots, traveling deep into the earth. Through this/these, we can rid ourselves of unwanted energy, and, like a plant, draw up vital energy from the earth for our wellbeing.
Centering can be achieved during meditation, where we find the center of our 'selves', that is, the silence. In the silence we are open to listening to the voices if our intuition, our guides, and our personal experiences of the divine. Hope this is of some help :)