A box and whisker graph is another term for a box plot. This type of graph consists of a box with the horizontal sides and vertical sides representing two different axes, or variables being measured. The data is represented inside the box as vertical rectangles that extended up and down to cover a particular range of data as required. The "whiskers" are the error bars. Uncertainty in the measured data being plotted is represented with these error bars which show how much the rectangles could be extended in either direction to accommodate the experimental error.
Photosynthesis is a light dependent process. Different wavelengths of light in the visible color spectrum will give faster or slower rates of photosynthesis in plants. In general, the red and blue parts of the color spectrum will give the fastest rates and the green part of the spectrum will give the slowest rate. If you did this experiment, you would measure the rate of photosynthesis of a group of identical plants each being exposed to a different particular wavelength of light. You would ideally repeat each trial at least twice to get more data to compare. You would then plot the information on a box graph with the wavelength of light on the horizontal axis and the rate of photosynthesis on the vertical axis. The range of rate data for each wavelength would then be covered by a rectangle for each wavelength of light. The error bars would represent the experimental error.
I cannot put a representative box plot in this answer, but one of the links below will show you a representative box graph.