What to do if you do not understand a lot coming up with a research interest? PhD admission
At least in the US, I don’t think most research advisors would expect undergraduate students to have the ability to understand everything in their published work. The idea is to be familiar with the general ideas, methods, and “top-level” results. If someone comes in with more than that, it’s a bonus.

In other countries, where the PhD program follows after a master’s degree, the expectations are naturally a bit higher.