Epa - what is meant by buying every year

I've just joined the CS and have an EPA-2 which I'm starting to pay into.

This would allow me to retire at 65, I also have private pension, my questions are

What happens to my pension if I retire ealier then 65, I know if I didn't take the EPA-2, all the pension would be reduced by day 4%. What happens due to the EPA?

I know people saying buying other epa every year - is this true, what does it actually mean Many thanks