hard to see much of the amp, but going buy the parts list on the inside of the tin, my knowledge of battery supplies and what I can see, I made a couple educated guesses. the schematic appears to be simplified somewhat, because it doesnt show any caps in signal at all and that doesnt seem to fit with the parts choice (cant see the rail splitter there) marked on the tin there are clearly several caps and we can see at least 4 of them and they are really too big to be signal caps and you NEED power supply caps; plus the TLE2426 'rail splitter'.
some opamps (the good ones) require a bipolar supply + / 'G' / - rather than a single supply + and Ground, both of these look the same to the opamp, but lets not go there. a bipolar power supply is the positive rail +, a ground reference of some sort G/0 and the negative rail -. batteries are a single polarity mostly and many battery operated devices lack access to a real ground. to generate a bipolar supply from the single supply a rail splitter is used to create and drive a ground reference, or middle point. this middle point is then used as the 'virtual ground'. for instance, an amp with a single 9v (normally 8.4v) battery, you would end up with
-4.5v / 0 / +4.5v but not really, really you still have 0v / 4.5v / 9v; but the opamp doesnt know this as its 'floating' inside the tin. with nothing to tell it otherwise 0v is 4.5v less than 4.5v thus -4.5, the reference 4.5v is deemed 0 and 9v is 4.5v above the middle, thus +4.5v.
this works great inside the amp, but once this clashes with a real zeroed ground reference outside the amp, the 4.5v DC offset that is now in the signal (because ground isnt where it should be) can damage equipment and has to be gotten rid of. headphones and speakers are designed to run off AC power and DC current is NOT GOOD. easiest, but not the best sounding way to get rid of it, like with IMODs and DIYMODs is with some caps at the outputs, DC cannot pass through a capacitor, only AC can. there are other methods for getting rid of DC and there are other methods of creating a split supply, but I spotted the rail splitter in the parts list