The average quality determines how many skill points your player will gain from the chosen drill. The higher average quality you choose, the more skill points your player will gain. But remember, the higher average quality you set, the more rest packs will be required in order to achieve the gains.
An average quality of 80 required less rest packs than 100. An average quality of 100 requires less rest packs than 120 and so on. Hope that helps, if not, please get in touch again.