When talking about languages, people sometimes say language A understands language B but language B doesn’t understand language A. How can that be?

Question

The way I see it, if one understands another language, it’s because of similarities in syntax, grammar, etymology or what not. But a similarity is by definition bidirectional, is it not? Is the idea that some languages understand other better than they are themselves understood a lie?

In: Other

Answer 1

every swiss german understands german perfectly, no german understands swis german unless they live there for several years.