Congratulations! You’ve learned a new thing. You’ve acquired knowledge, or maybe even wisdom.
But! Now that you know your precious new thing, as time passes, you’ll be less and less able you’ll be to understand the mindset of someone who doesn’t know it. This is true for everyone who knows the thing!
So you’ve actually become a really good candidate to teach the thing to somebody who doesn’t know it.
And! When you don’t understand the lecture, maybe you should ask the student next to you to explain it to you, rather than the professor.