A setter is a method which sets a value for one of your parameters. E.g. many people say it's not nice to have a public variable in a class:

public class SomeClass {
     public int someInt = 0;
}

now you can access the variable someInt directly:

SomeClass foo = new SomeClass();
foo.someInt = 13;

You should rather do something like that:

public class SomeClass {
    private int someInt = 0;

    // Getter
    public int getSomeInt() {
        return someInt;
    }

    // Setter
    public void setSomeInt(int someIntNew) {
        someInt = someIntNew;
    }
}

and access it through:

SomeClass foo = new SomeClass();
foo.setSomeInt(13);

All this is just convention... You could name your setter-method however you want! But getters and setters are a good (and readable) way to define access to your class varaibles as you like it (if you want to make it read-only you could just write the getter, if you don't wan't anybody to read the value you could only write the setter, you could make them protected, etc...)