What is the difference between population size and population density?

There's roughly 2000 pandas left in the world so this is the population size of pandas. The population density is when you divide the number of pandas between a given area. So the population density is the average number of animals per unit of area.

2017, all pandas are moved to China + Russia, Population density of pandas in China + Russia is very high

If, in 2017, all of the pandas in the world are moved to China and Russia, as shown in this drawing (the pandas are purple dots), then the population density of pandas in that area is going to be fairly high. The 2000 pandas in the world are now concentrated in one area so this area has a high concentration of pandas.

Now another situation

2018, all pandas previously in China + Russia are moved all around the world, meaning the population density of pandas in China and Russia is very low

Here, in 2018, the pandas decided to go on a road trip. 2000 pandas have been distributed around the world evenly so Russia and China would have say, 10 pandas instead of 2000 in 2017. This would mean that in 2018, the concentration of pandas in China and Russia would be much less and therefore the population density of pandas in China and Russia in 2018 would be less that in 2017 because the pandas are more spread out.

Hope this helped :)