I agree with pm133, read as much as possible without stressing yourself. You are not expected to know everything but knowing the basics would be a great help. I would also encourage you to read broadly and go over the stuff early in your undergrad. Having a firm foundation before you start is good way to go forward with your PhD. A few months into my PhD, I realized I had to relearn an entire topic from my undergrad because my memory of it wasn't good enough (even though I got 82% on that module), which was a drag.