There are two ways to connect your computer to your audio, analogue or digital.
The sound card of the computer does the conversion to analogue.
You connect the analogue out of the computer to the analogue in of the amplifier.
Often the sound quality of the onboard sound card is only moderate.
In case of a desktop, you can add a discrete soundcard.
In case of a laptop you can't so a digital out is the only way to improve on it.
This requires the PC and the DAC to have a digital protocol in common.
Popular protocols are:
SPDIF is the standard in the audio world to connect audio equipment digital.
This standard is less common in the computer world.
There are sound cards having SPDIF out.
You connect it to a DAC or a receiver with digital in.
USB is the standard in the computer world
More and more audio manufactures are producing high quality audio components with a USB input.
As USB is the standard in the PC world and SPDIF in the audio world, a USB to SPDIF converters can bridge the gap.
Streaming audio players use the network to connect to a server.
It all started on the internet (internet radio) but if you can play music located on a server somewhere on the internet, you can use the same technology to play the music using a PC or a NAS as a server in your home network.
Almost all modern AV equipment supports the DLNA standard.
All equipment supporting this standard can communicate with each other.
Modern TV’s can show the pictures on your PC, your smart phone can used as a remote, etc. etc.
Why? All are networked and all are DLNA compliant.