Meaning of this in specific situation

It's necessary to specify that you want to assign the value to the field, rather than the parameter or local variable:

public void setFoo(Foo foo) {
    this.foo = foo;
       ^        ^
       |        \--- Take the value of the parameter
       \---- Assign to the field
}

If you just wrote:

foo = foo;

in the above, then it wouldn't do anything - it would be assigning the value of the parameter back to the parameter.

Another option, however, is to use a different parameter name instead:

public void setFoo(Foo newFoo) {
     foo = newFoo;
}

Now the field and the parameter have different names, so you don't need to find another way to differentiate between them.