Calling someone "hot" means that they're attractive. Calling a woman "smokin' hot" means that she is very, very attractive.
Being "smokin' hot" is a little different from being, for example "gorgeous". A "smokin' hot" woman is attractive in a sexual way. A "gorgeous" woman has a beautiful face and might be very well-dressed.
It is possible to describe a man as "smokin' hot", but much less common.