Are the variables wi, pdf, x, means to be scaler type or vector type here?

Why is there a -wi on the second to last line? What does lights.radiance do with the incoming light going in the opposite direction?

Nevermind, I figured it out: it's because intersect(x, wi) is giving you the intersection of the ray wi from point x to the light source since Ren stated by convention wi is pointing away from the surface. for radiance, we want the light ray pointing towards the surface so we have -wi.

@skyscrapers1999 Not sure what you mean by scalar/vector type, but $w_i$ is a direction in a hemisphere around a point, so it is defined by two variables, $\theta$, $\phi$ like with solid angles. $x$ I believe is the position of the point, so it can be defined as a vector in 3D. The pdf is just a scalar value because we are looking at a uniform distribution. It just so happens the area of a hemisphere is equivalent to $2 \pi$ in solid angles, which makes the pdf of any point $1 \ 2 \pi$

Thank you.