I'm not Korean but here it goes anyway. 알다 is indeed to know but 아세다 isn't a verb it's actually the verb 알다 plus the honorific suffix -시, though the ㄹ gets dropped in the process. 알시어요 becomes 아세요 So for example : 나 알아 ? : Do you know me ? (Impolite form) 저 아세요 ? Do you know me (polite honorific version). Again I'm not a native speaker hope this helps though 😄

Also not a native speaker, but as another person said 알다 + honorific (으)시 becomes 아세요. This is because of a spelling change where ㄹ before ㅅ is sometimes dropped. Here's another example: 살다 -> 사세요 To live -> lives (honorific)