contour plot of EL indensity

I am study degradation of PV Cells by measuring the chang of electroluminesence(EL) during the time. I have a lot of images I have taken them during 2 day. My friend help me to make contour plot of EL indensity of the images for every 1 hour. I attached this Figures. However, I have diffecality of explain these figure because I never work with contour plot befoure. I didn’t uderstand what these # in graph means. I need help understand and explain these Figures.