Rabulias
the Incomparably Shrewd and Clever
I read that to mean that if I am shooting at a target from 10 feet behind a low wall, the cover counts for my attacks against them, and their attacks against me.PHB p.196 said:A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover.
If I am adjacent to that low wall (or an arrow slit, as has been discussed) so that I can get the release point of my arrow beyond the cover, then it does not apply for my attacks against the target, but it still provides cover for the target's attacks against me.