Discuss here mixing and mastering

Basically it's the attack at the beginning of a sound, like the stick striking a drumhead or a pick hitting a string. As opposed to the sustained tone that comes after. So sometimes you want to tame that with compression for a more even sound, but you also need them eg for drums that should 'hit' viscerally and not stroke your ears like a feather.

got it, thanks - the only other thing I knew was "that's what Rush fucked up on that album that people hated"

Haha, oh really?

https://gearspace.com/board/mastering-forum/736700-cant-stand-new-rush-album.html

Vapor Trails, I think