大事(dà shì) - a big deal, a major event, an important matter in Mandarin

大事 (dà shì) is a matter that carries weight: a milestone, a crisis, a piece of news that changes things. Its natural partner is 小事, a trivial matter, and the pair are used together constantly to weigh one thing against another.

Two set phrases are worth banking early. 出大事了 is the shout that something has gone badly wrong, and 不是什么大事 is the shrug that means never mind, it is nothing.

The English-speaker trap is the measure word. Because 大事 is countable you need 件, not 个: 一件大事, 两件大事. Learners default to 个 for everything and it sounds off here. The other trap is reading 大事 as literally a large thing, which would be 大东西. 事 is an abstract affair, never a physical object.