A DAC is a peice of hardware that takes the digital signal from your computer (skipping your sound card/internal audio) and converts that in to an analog signal that your headphones can understand. An amp amplifies that signal (louder).

If you get worse or quieter than ideal sound from your computer you might benefit from a DAC/amp as it would bypass your computer's audio hardware

You might be served well enough by a dongle or want something else. HD600 isn't particularly hard to drive but it's high impedance might be an issue

Is this still accurate? I remember an old design frying gear and Schiit being coy about it but that was years ago