groundwork

so everyone's solution to any horse problem ever is "more groundwork" and i agree with that, but...

what IS groundwork? everyone mentions it as some cure all for any behavioral issue but never gets into specifics. i'd like to look into doing more ground work as 70% of the time i spend with my horse is lunging and on the ground due to some saddle issues.

does anyone have specific exercises they recommend? or websites? how do i start?