Solidity – Understanding the Purpose and Usage of block.coinbase in Smart Contracts

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) 
    address(x).transfer(10);

And here is a some kind of definition (from this doc):

this: the current contract, explicitly convertible to address

In short now is just an alias for block.timestamp and it is the number of seconds since the Epoch as per documentation.

Beware that this value is set by miners so there is a little potential for a malicious manipulation but general nodes are meant to coordinate.