It means that the meatball has salt in it, or simply that there is salt in the meatball. Apart from the "there is... " structure, this is the most popular way to say something like that in Greek, which, I agree, does not sound natural in English. When something is salty, είναι αλμυρό.
Hmmm... I hadn't thought of that! From what I can deduce, it's the -ρος ending that does the trick in Greek, e.g. ζουμερός - juicy from ζουμί (old/formal is ζωμός), φανταχτερός, μοχθηρός from μόχθος, φθονερός φθόνος, αστραφτερός. The vowel before -ρος depends on the root. And of course not all words with that ending have that -y meaning (and maybe some of my examples above are wrong): comparative adjectives end in -ρος as well. e.g. νέος/νεότερος - new/newer.