Some people are “mixed” between houses, even though that can’t actually happen in Hogwarts. Basically, they have the traits that can match in both houses.
For example, Slytherclaw is a mix of Slytherin and Ravenclaw. The person who is in Slytherclaw would have both Ravenclaw and Slytherin traits.
Being a Slytherin isn’t a bad thing at ALL. Not all Slytherins are evil, and some Slytherins turn out to be really really really amazing people- Draco Malfoy is proof of that-