"Full sun to part shade" is a designation for types of plants that tolerate up to six hours of sun each day, but need some shade, particularly in the afternoon. Plants that require full sun need six to eight hours of sunlight and tolerate the afternoon sun well.
It is important to pay attention to the sunlight needs of particular plants in order to grow them successfully. A plant that receives too little or too much sunlight is prone to wither and die. No exact formula exists for determining just how much sunlight a given plant needs, but professional growers have devised categories for particular types of plants. Plants that get a great deal of sunlight often require more watering, as well.