I just started reading a paper "On the Langlands correspondence for symplectic motives" by Benedict H. Gross, which talks about Symplectic Motives. The paper starts with the following

Let $M$ be a pure motive of weight $-1$ and rank $2n$ over $\mathbb{Q}$, with a non-degenerate symplectic polarisation

$$\psi:\wedge^2 M \to \mathbb{Q}(1)$$.

I want to understand this line. I know what is a pure motive of certain weight and rank over $\mathbb{Q}$. What I don’t understand is what is this map $\psi$, which is referred to as the non-degenerate symplectic polarisation.

Thanks in advance for any kind of help.