Hi Nelson,
This is Old Scots, so would never be used in a real life context. I imagine you read this? The sentence above is from a poem by Scottish Poet Robbie Burns.
The meaning would be roughly 'you are a very beautiful girl'
These days some of this words would still be used in the Scots dialect.
Bonnie = beautiful
lass = girl
Here 'fair' would also mean pretty or beautiful.
Christine