A streamer gets your music from your wifi router (modem) to your DAC or you can combine both functions in a streamer/DAC. You can connect to your wifi router wirelessly (wifi) or wired (ethernet cable, i.e. Cat 6, Cat 7, etc.).
Digital inputs and outputs will be labelled as such on the component. There are several types of digital cable that you can use to connect a streamer (digital out) to a DAC (digital in).
If you want to read up on this topic, there are a number of very recent threads about New to Streaming, Newbee Question About Streaming, etc. Just do a search of the forums for Streaming and sort by date.
If you read those and are still puzzled, you’re like me. The easiest way to learn about streaming is to buy a Bluesound Node 2i (a Streamer/DAC with a digital out so you can connect it to a separate DAC, if you want to) for $550 list price. It’s really easy to set up and the BluOs app you use to control the streamer is very user friendly. It will sound good enough for starters and you can get a separate DAC if you want better sound once you decide whether you like streaming or not. Eventually, you can spend as much as you want (or have) for better streaming gear if you feel that’s necessary.
Tidal, Qobuz and Amazon Music are streaming services which cost $15 - $20 a month. I think they all offer a free one month trial so you can decide which one you prefer or if you like streaming at all. I like streaming, other people like physical media. You can do both, using streaming to find music you really like, then buying a physical copy.