A DAC is a digital to analog converter, or a device that converts a digital audio signal to an analog waveform. You may already know that you need an external DAC, but should you be looking at a dedicated DAC and amplifier, or a DAC/amp combination?
What is the difference between a DAC/amp and a regular DAC?
A DAC is a device that converts digital audio to analog waveforms, and when you purchase a dedicated DAC without an amplifier (like the Schiit Modius), the device does just that. Usually a dedicated DAC will have some outputs on the back, like Unbalanced RCA or Balanced XLR connectors, but they're designed to be plugged into an amplifier.
An amplifier is needed to convert the analog waveform into something that's powerful enough to actually cause your speakers or headphones to output sound that's loud enough for you to hear. So, a dedicated DAC, even if it's outputting an analog signal, will not have enough power to actually power your headphones or speakers (though if you have powered speakers, you can use a DAC-only with them, more on this later).
This is where a DAC/amp combo comes in. It's essentially a DAC with built in amplification circuits to output enough power to drive an audio device. Usually with a combo DAC/amp, it's specifically for headphones. Standalone speakers take a lot more power, so it's less common to see a small DAC/amp combo that's able to power them.
For headphones, a sufficiently powerful amplifier is important. Some headphones are notoriously hard to drive-- meaning that not all amplifiers have enough power to actually send a loud enough audio signal to drive the headphones-- and a high quality DAC/amp will ensure that you can use any pair of headphones you want, even those hard to drive ones you purchase in the future.
Do I need a DAC/amp combo with powered speakers?
No, you don't. Powered speakers have a built in amplifier inside of them, meaning that they will accept the weak audio signal directly from the DAC-only and amplify them to an audible volume. But remember, if you don't have an actual DAC/amp combo, you won't be able to plug in headphones!