Object-oriented programming (OOP) is a style of programming where you organize code around objects rather than around functions. An object bundles data (what it knows) with behavior (what it can do).
Imagine a BankAccount. It knows its balance and ownerName. It can deposit() and withdraw(). Without OOP, you'd store the balance in a loose variable and pass it to separate functions. If you had accounts, tracking which balance belongs to which owner becomes a nightmare.
With OOP, each BankAccount object carries its own data. You call account.deposit(50) and the right balance updates automatically. Java was built from the ground up around this model.