English has only "my", but many other languages have a word for each gender.
French: Mon (masculine), ma (feminine), mes (plural). Portuguese: Meu (masculine), minha (feminine), meus (masc. plural), minhas (fem. plural).
It happens that Russian also has the gender neuter. And яблоко is neuter.
Мой (masculine), Моя (feminine), Моё (neuter), Мои (plural).
So if you want to say "my apple", it's "моё яблоко". But in plural, it's "мои яблоки".
And here I'm only talking about the nominative case. Multiply by 6 to have the rest.