What do they mean when they say 'Smart Casual'?

Think of it as more like ‘smart wear’. Essentially with smart casual you do not have to wear a suit, but it is not as casual as blue jeans and a jumper. You can wear jeans, but stick to black or a very dark navy. They should look professional and smart, and if you do choose to wear jeans then make sure you pick a smarter shirt or top.

If you are stuck, read;

Maarten Karremans