That plot is the version of heat map. You can see that each line represents the 5-year interval (from 0 to 90). The first (bottom) line seems to represent the data on newborns. Each column represents a year. The colour of the cell means something related to residuals (maybe average residual). For further details legend and context of the research is needed. Actually, we are grouping our residuals by age and year to inspect the model. Ideal model should give completely random picture without any trends or relationships. If you look at the distribution of colours by year - no regularities can be found. But with age we have completely red and white cells in 0-5, 5-10 and 10-15 intervals. That means that model explains children mortality worse than adult ones. There may be many reasons for that. For example, it can be unbalanced data with few observations for children and many for adults or there is some additional factor not included in model that has effect on children mortality etc.

Thank you so much Oleg. This answer is really helpful for me.