In addition to what others have said, my understanding from Team Cherry's few reveals over the years since HK released is that Silksong takes place in a different location than Hallownest, the setting of Hollow Knight (though within the same world). Thus, given that most of the story of Hollow Knight is about Hallownest and what happened there, it is likely that you wouldn't miss much besides a few cool little connections by playing Silksong first.