Learn Russian: Мы накупили цветных шариков и сладких угощений. - We bought a lot of colorful balloons and sweet treats.

This is due to the prefix на- in the verb накупили. When на- is added to a verb to indicate accumulating a large quantity of something, the verb requires its direct object to be in the Genitive case rather than the Accusative.

Does накупили mean exactly the same thing as saying купили много (bought a lot)?

They mean roughly the same thing, but накупили is more expressive. While купили много simply states a fact, накупили emphasizes the action of gathering an abundance of things. It often carries an emotional or colloquial tone, implying you went all out or stocked up.

Why does the Genitive plural of угощение end in -ий (угощений)?

Угощение is a neuter noun ending in -ие. A reliable rule to remember is that neuter nouns ending in -ие (like здание or занятие) form their Genitive plural by changing -ие to -ий. Therefore, угощение becomes угощений.

The word шарик literally means 'little sphere'. Is it the standard word for 'balloon'?