Requirements for Acceptance

I'm applying for Early Childhood Studies at Aberystwyth as an American high school senior, and I got accepted with conditions!!! Very exciting (it is my top choice for uni), but when looking at the conditions for being actually accepted, it says "This offer is subject to you obtaining Pass Access with Merit in minimum 50% of level 3 units" and I'm not sure exactly what they mean by that.
Is there anyone who can translate this into the US school system terms so I can know what to try to graduate with?
(btw I'm not taking any honors/AP classes, and it's too late to get those credits, but I have really good SAT scores and generally good grades)