The quote is attributed to Feynman, but it was repeated many times by many professors I took classes from in undergrad and graduate physics.

There are many areas of physics that have been developed in the past century that would be incomprehensible to a physicist back then... of COURSE they would at least have the tools to begin the long process of catching up on the developments that lead to these new ideas, since the cutting edge of physics must necessarily build on the reality previously established.

But not all areas or concepts in physics can be truly understood by the human mind in the same way as others.

I got to a point where I 'understood' newtonian mechanics- I had a good intuitive feel for it, could come up with the right answer in multiple ways- It made sense and I would know qualitatively what the answer would be before doing the pages of math.

Classical electrodynamics also made sense to me- the math was more difficult and a little harder to visualize- but I could see the beauty of it and use it and understand the process and the answers...

Special relativity was harder to feel comfortable with, but eventually I got there, and it was my favorite subject, General relativity I found beautiful, and loved it- I always felt like I was on the verge of understanding something very fundamental about the universe- but it was elusive- for a long time I seriously considered going into pure theory- focusing on general relativity and cosmology- in the end I found the hands on discovery of experimental physics more enticing.

I took many quantum courses, and did well enough but never felt that I understood it- at best I got to a point where I could follow the process, and get an answer- I developed ways of 'visualizing' what was going on, but never believed the visualizations were anything more than imperfect metaphors for the 'truth' of what was going on...

Quantum electro-dynamics and particle physics I hated- It was like having a recipe book- extremely detailed and fixed rules for combining the ingredients- you could use the recipes to do wonderfully complex and surprising dishes that were without a doubt turning out exactly as they should... but when I was done I still did not feel like I understood anything about the ingredients, or even the true nature of the dish I had prepared according to the recipe...

Entanglement, wave-functions, things that are both particles and waves and neither- the human brain may never be able to truly 'understand' the true reality of it- at best we will come up with precise mathematical formulations that exactly calculate the processes in question, and imperfect metaphors and analogies we tell ourselves to 'understand' the meaning of it all.