Answer:
It means that when people get into trouble, for example financial trouble, they may be "backed into a corner" and feel forced to do things they wouldn't normally do in terms of survival.
For example, suppose that you get lost in the woods. After a few days without food, you become desperate for a meal, and so you may find yourself eating plants, bugs, animals, fungus, etc. that you wouldn't normally eat just to survive. This portrays the meaning of the phrase "desperate times call for desperate measures" - the desperate time, or desperate situation, is that you are stuck in a forest without any normal food or means of obtaining such food; the desperate measure is the action plan you follow, namely, to eat anything that is edible until you can afford to eat normally again (i.e., when you find your way out of the woods).