Polish ditinguishes between perfective and imperfective verbs:
jeść [imperfective] - zjeść [perfective]
So this sentence means something like: "I want to have eaten something good".
"Chcę jeść coś dobrego" would mean that "I want to be eating something good [now]" or "I want to eat something good [regularly]"
Often one English verb can translate to 2, 3 (or even more) different verbs in Polish, depending on tense (and additional adverb) used in English phrase. See "Aspect of Verbs in Polish, Verbs of Singular, Multiple and Completed actions"
I think dobrego is an adjective describing the noun coś, which appears to be in the Accusative. Is dobrego in the Genitive, or is it actually Accusative masculine personal or animate? If Genitive, why would that be? If Accusative, what makes it personal or animate as opposed to the expected inanimate (assuming the something you want to eat is not still alive and kicking :-)
"coś" is neuter http://sgjp.pl/leksemy/#90003/co%C5%9B - more precisely, n2 = 2-nd type of neuter i.e. it takes cardinal numerals, while nouns type n1 take collective numerals https://en.wikipedia.org/wiki/Polish_grammar#Numbers_and_quantifiers