Balance

$P_w$ is the equilibrium price level set by the supply and demand of the international market.

Exports

The excess quantity supplied, $Q_s-Q_d$,, will be exported. There is welfare gain since the producers get to sell more at a higher price. However, the consumers lose since they are getting less at a higher price. Nonetheless, the overall effect is a welfare gain.

Imports

When the country opens to international trade, there is a price at $P_w$. At that price, the country can only produce at $Q_s$, while the people demand $Q_d$. So the government imports $Q_d-Q_s$ .