1) To be really good.
2) As replacement to okay or great.
Note: Usually said with half-hearted meaning or sarcasm OR with little or no interest in the subject of the conversation.
Boy: Sir, is this alright?
Mr. Lowis: Yeah proper smickles
Boy: How would you answer this question sir?
Mr. Lowis: If you need to ask that after all these lessons you are clearly not paying much attention.
James L.: Sir it's easy! You're just stupid!
Mr. Lowis: Laughter
Boy: So would it be alright if I answered it like this?
Mr. Lowis: Yep, proper smickles.