Arasseo (알았어) is the past form of alda (알다) "to know" and means "I understand," or in some cases "I knew." When used as a question (알았어?) it means "Do you understand?" in an informal or possibly impolite manner, depending on who you are speaking to and how you use it.

Yeah, 알았어 means I know, or Okay. When someone says, 알았어요, they are pretty much saying "Okay, I get it, I know already." But when someone asks you that, they are pretty much saying "Okay? Do you know already? Did you get it now?" It's pretty simple. When you keep Koreans say arrasseo, you'll catch on. :)

알았어요 ? means do u understand? in korean btw u look pretty.