Wow! What a broad subject to try to condense into a few paragraphs!
The best description I’ve heard is that nutrition is providing your body with the vitamins, minerals and herbs necessary for self-healing and self-maintenance. I would also include that it would be eliminating the things from your body that would be detrimental to the process of self-healing and self-maintenance.
Fresh fruits and vegetables, regular moderate exercise (exercise that is too strenuous actually creates free radicals in your body), drinking lots of clean water and eliminating carbonated beverages and products high in sugar is a great way to start.
There you go. If most of us would follow this simple path we would be healthier that the majority of our friends, relatives and neighbors.