Learn English – English equivalent of Polish idiom “To explain [something to someone] like a cow in bound”

The most common figure of speech in a situation like the one you describe is to say that something (a conversation, a topic, or especially a joke) is (or goes) over someone’s head, as in sense 1 of this Oxford Dictionaries entry:

Beyond someone’s ability to understand:
the discussion was over my head

If the person who does not understand the conversation is the one saying it, it will often be in a slightly self-deprecating form, such as:

Woah … this is way over my head.

If others say it about him, they are quite likely to come off as fairly condescending.

From the notion of something passing over one’s head come two closely related gestures that are often used together:

1. Moving one of your hands quickly over your head (from the front to the back), as if imitating something physically flying past you above your head; and
2. Saying, “Whoooosh!”, imitating the sound of someone whooshing past very close to you.

Once again, this is seen as slightly self-deprecating if done by the person who does not understand, and definitely offensive if done by others.

An alternative to expressing that something goes over one’s head is to state that one is out of one’s depth (sense 1.2), an extended sense of a phrase meaning ‘standing in water that is too deep’:

In a situation that is beyond one’s capabilities:
they soon realized they were out of their depth in Division One
I find it difficult to talk in a situation like this—I’m out of my depth

This is a bit more polite and less self-deprecating; it’s also less of an idiom and more of a straightforward collocation. Something like the following is a quite polite way of saying that you don’t understand the topic:

I’m afraid I can’t follow you. I’m a bit out of my depth in this conversation.

There's no cure for stupidity.

Comes close to the connotation of the inevitability of nature the Polish one has.