In object-oriented programming, it is the relationship between classes of objects. It allows one object instance to cause another to perform an action. This is known as “sending a message” or “calling a method”
In programming, there are different relationships that can be used between your objects. Association is known as a weak relationship because the objects don't necessarily have to depend on each other.
Explanation / What does this mean
Similar to the relationship between a Teacher and a student. A student can go to multiple teachers and a teacher can have multiple students. There is no "owner" in the relationship and both objects can live independently from each other.