Tyler says his song needs "mastering" before it goes on Spotify... what on earth is that and why does it cost $50??

So Tyler and the garage band finally finished their song... the one they've been banging away at in his mom's garage since August... and now he tells me it has to get "mastered" before they can put it online. He found some fella on the internet who'll do it for $50 and he was sort of hinting that Grandma might like to

mastering is the last polish step after the song's mixed someone with good ears and good speakers tweaks the overall EQ, compression and loudness so it sounds right on earbuds, car stereos and laptop speakers, and doesnt come out way quieter or muddier than everything else on a playlist. they're working on teh finished track, they dont fix the actual playing. $50 a song is normal, cheap even big-name mastering engineers charge a lot more. so its probably not a scam, but ask the guy for before/after samples of work he's done. he doesnt strictly need it either. spotify turns everything up or down to roughly the same volume anyway https://support.spotify.com/us/artists/article/loudness-normalization/ and there are

like,

...last polish step after the song's mixed someone with good ears and good speakers...

yeah pretty much, but it CANT save a bad mix. if your vocals are buried, mastering just makes them louder and still buried lol. also on an album they match the levels between tracks so song 3 doesnt randomly blast you i'd tell you a mastering joke but it needs more polish