Let $\mathfrak g$ be a Lie algebra over $\mathbb R$. I need to understand the following statement I found in a book (Discrete Subgroups of Lie Groups, Raghunathan p.123): the cohomology $H^p(\mathfrak g,\mathbb R)$ is defined by considering $\mathbb R$ as a module over $\mathfrak g$ via the trivial representation.
So what does that mean? And how do the $\mathfrak g$-submodules of $\mathbb R$ look like?