# notation of derivative probability function

I am reading article about bayesian predictive function. In the article it denote posterior distribution $$\pi_n(d\theta) = \frac {\prod^n_{i=1}f(y_i|\theta) \pi(d\theta)}{\int \prod^n_{i=1}f(y_i|\theta)\pi(d\theta')}$$ where $$\pi$$ is prior for $$\theta$$ and $$\pi_n$$ is posterior given $$(y_1, y_2, ... ,y_n)$$. In the addition, $$Y_1, Y_2, ..., Y_n$$ are iid. The article also denote predictive density like that $$p_n(y) = \int f(y|\theta)\pi_n(d\theta)$$. But I don't know what the $$d\theta$$ means in this case.

Thanks for everyone who will help me.