A term to define a person (most often used to describe a woman) who is very beautiful and good at what they do, such as acting. There are many other examples but the most common types of people who 'slay' are actors because they are considered more popular amongst everyone else.
The term also has different terms(Slay, slaying, slayed, etc)
Bro, have you seen that hot actress? She slays I love her!