According to this guide:
Contracts Inherit all Members from Address.
It means that
this is the pointer to the current instance of the type derived from Address (in your case - current instance of Helper), and
balance is a member of Address.
It helps you distinguish between contract's own balance and any other balance, like in this example:
address payable x = 0xFFfFfFffFFfffFFfFFfFFFFFffFFFffffFfFFFfF;
if (address(x).balance < address(this).balance)
And here is a some kind of definition (from this doc):
this: the current contract, explicitly convertible to address