What it means
Your main quest is the thing that actually matters, your real goal or priority, as opposed to all the side quests and distractions pulling at your time. The reminder is simple: stay on the main quest.
Usage examples
"Finishing the degree is the main quest, the parties are side quests."
"I keep getting distracted, I really need to focus on the main quest."
"Answering emails isn't the main quest today, getting the rent sorted is."
"Gym, meal prep, early night. That's the main quest. The random pub crawl can wait."
Where it comes from
It comes straight from gaming. In loads of video games, the main quest is the central mission that moves the whole story forward, while side quests are optional extras. Online and in everyday chat, people started using it for the real-life priority that actually matters when everything else is trying to nick your attention.
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