How it works?
Let start with the key word public. In real life when something is public it means that we can use it without permissions from anyone. We also don't need to tell anyone that we want to use it. For example in Amsterdam (capital of Holland - county in Europe) it is very popular to use public bicycles. These bicycles are public (and free). You can take one and drive from one point of city to another. You don't need to ask anyone for permission. In OOP we have a very similar situation. We can say that property or method is public. It means that there is no restriction to use it. We can use it everywhere.
public method borrowBicycle()
$bicycle = new Bicycle(); // assume we have also class Bicycle defined somewhere earlier
Absolutely no restrictions on method borrowBicycle(). Everyone can borrow a bicycle.
So please remember: PUBLIC = NO RESTRICTIONS