Graph of loss function in high dimensional space

In lecture 2, Jeremy sir mentioned that in a high dimensional space (for a large no. of parameters), the graph of the loss function plotted against the parameters has a lot of flat points, and also some sharp ones. Is there a reasoning behind this because what I know of it is that it is difficult to visualize a high dimensional space of 4 or higher dimensions. So, how can he be that confident in his assertion? Maybe some external links may help.