It means nothing, I'm sure people say it means bad luck but its not true its only bad luck when you give it the power for it to be bad. Just think people own cats everywhere and nothing bad happens. So it means nothing.
It could mean they smelled food in your house and were attracted to it, or they are looking for a warm place. Cats like very warm places. Or they could be looking for a mate, if you have or have had a cat in your home before that they can still smell the scent of. Or it could just be the curiosity of cats; they like to explore new places.
In my opinion, I think it means that it is either hungry or it a sign that it needs a home. If it did actually happen to you, then give it a home. But if you hate cats, find someone that will love it.
It probably is a female cat. The cat may be pregnant and needs a warm cosy place as shelter for itself and for its offsprings.
Oh kitty why were you afraid when i come..