Encapsulation means hiding an object's internal data and exposing only what's necessary through methods. In practice, you mark fields private and provide public methods to interact with them.
Why bother? Suppose balance is public. Any code can write account.balance = -500;. Now you have a negative balance with no validation. If balance is private, the only way to change it is through a method you control, like withdraw(), where you can check for negative amounts.
Encapsulation gives you a single place to enforce rules. Every access goes through your methods, so bugs have fewer places to hide.