if $f:X\to Y$ is a map of small v-stacks, Scholze and Fargues define relative homology as the left adjoint to the $f^{\star}$. They say the left adjoint exist because $f$ is a slice in $v$-site (they actually take a solidification of this left adjoint). I have no idea what this means and why can't we do the same in the étale site? Perhaps we can do this but the result would not be constructible?

So my question is what is a slice in a site, why does it give a left adjoint and why can't we do this in the usual setting?