Okay, imagine you really want ice cream but your mom says you can't have any because it's too late at night. However, she tells you that she has something that's almost like ice cream: frozen yogurt.
That frozen yogurt is like ersatz ice cream, which means it's a substitute or replacement for the real thing. Ersatz basically means something that is kind of like or pretending to be something else.
So, whether it's a fake sugar substitute or a toy that's trying to look like a real animal, it's all ersatz. It's almost like the real thing, but not quite.