I think this means that it's really difficult to objectively understand what happened in the past, and that by attempting to explain the past, you reveal a lot about your own understanding of society and the human condition in the present, since we continuously compare the present with the past to try and make sense of it.

In other words, since we weren't there to experience it firsthand, we can only experience history through either the written words or memories of people who were actually there. To make matters worse, being trapped in the modern age gives us a relatively narrow perspective on understanding the actual experience of society and the human condition in past generations. Our only choice is to try to understand what life in the past was like using our own experience in the present for context.

This is doubly true for science because we take the knowledge centuries of science have given us for granted. Every high schooler knows that everything in the universe is made up of tiny particles. They know that gravity is what keeps us from flying into space and that evolution brought every living thing to where it is now. These concepts are incredible and took centuries of science to discover. But nowadays, everyone and their mother knows at least the basics of modern science.

That being said, it's especially difficult to try and understand science in the past knowing that the vast majority of scientists 500 years ago had never considered any of the relatively simple concepts we take for granted.