The jesters are always expected to be silly and humorous at a moment's notice. 'At a moment's notice' means instantly when given a signal.
The jesters are expected to be silly in consideration of the crumbs that fell from the royal table. I think that this means that the jesters have to be silly and humorous because they care about the crumbs that fell from the royal table. The royal table is for royalties so the food on that table are first place. The jesters might feel hungry or long to be like royalties eating as much as they want. When they see crumbs fall, they see good food wasted as the crumbs touch the floor.
So the jesters actually have to be ready and prepared to be silly and humorous instantly when given a signal because they long to serve the royalties well and hope that they may be favored since jesters are used for entertainments in courts so they need a lot of energy to entertain the royalties and give downright silly remarks.
***Not a teacher***