A trait is a collection of methods.
Data types can implement traits. To do so, the methods making up the trait are defined for the data type. For example, the u256 data type implements the Into trait. This allows a user to write 1.into() to convert a felt into a u256.
In this way, traits are somewhat similar to Java interfaces and C++ abstract classes.
Because traits indicate shared behavior between data types, they are useful when writing generics.