A component that pulls music from streaming services or your home network and feeds it to a DAC or amplifier, controlled from an app.
A streamer is the modern source component: it connects to your network, accesses services like Tidal, Qobuz and Spotify (plus files on a NAS), and outputs either a digital signal to a separate DAC or, if it has one built in, an analogue signal straight to your amp. The good ones live or die on their app and reliability as much as their sound.
Because bit-perfect data delivery is largely a solved problem, the audible differences between competent streamers are small — most of the sound quality is set by the DAC downstream. Spend on the DAC and the app experience; don't overspend chasing a "musical" ethernet bridge.