a product of two relations $R,T$ element of $A \times A$ is defined as? I don't understand?

Question

$[RT = { (a, c) | \exists b \in A : (a, b) \in R \wedge (b, c) \in T }.]$

so i dont understand why a product of two relations is defined like that? product stands for * , so shouldnt it be a Cartesian product of two tuples? or do i understand the definition of a product of a cartesian product wrong?