He defines a graded ring as $R$ together with a direct sum decomposition
$R = \bigoplus_{n \in \mathbb{N}}R_n$
where $R_n$ are abelian groups such that $R_iR_j \subset R_{i+j}$
I understand addition on $R$ i.e. $f,g \in \bigoplus_{n \in \mathbb{N}}R_n$. Then, $(f+g)(n) = f(n) + g(n)$, but confused about how multipication is defined i.e. what is $fg$?