If the screen says "Play" rather than giving a price, that means either that
- You are looking at a page for the Apple Music subscription service, or
- You are looking at a page for the iTunes Store – and it knows that you have already bought the song. If it knows you have bought the song, you'll probably be able to download it again.
Redownload music from the iTunes Store on your iPhone or iPad - Apple Support
Redownload music from the iTunes Store on your Mac - Apple Support
Redownload music from the iTunes Store on your PC - Apple Support
Presumably the iPod in question is an iPod touch – since none of the other iPods ran iOS or were able to directly access the iTunes Store. With them, you would get music onto your Mac or PC, and then synchronize the music down to the iPod over a cable.