"Optional sidequest" refers to a task or mission in a game or story that is not necessary to complete the main objective. It is up to the player or participant whether they want to engage in this additional activity. Full definition
I hit level 36 and achieved the «S» ending in sixteen hours and thirty minutes, but that was by pushing past a large chunk of optional sidequests in the back half of the game. (kotaku.com)
There are numerous stars and levels that the player does not have to complete to beat the game, but still exist close enough to the normal game pathway to qualify as regular game pieces and not optional sidequests. (superluigibros.com)
Another of the game's systems is the «bonds system», in which characters can partake in many optional sidequests with non-player characters. (amazon.com)