That sounds like a problem to me. I don't know your situation, but when this happens to me, I am blunt and assertive that I don't understand what they're saying, like "I did not follow any of what you just said. I have no idea what X and Y are, and my understanding of Z is... Is that correct?"

in the short term people may think less of you, but in the long-term you will actually learn this stuff assuming you have a helpful mentor. If the senior engineer's job is to break things down for junior folks, and you're not understanding their explanation, then they're not doing their job.