"Her wine is good."
Translation:Jej wino jest dobre.
I'll try to rephrase vytah's comment. Historically there have only been possessive pronouns for the first and second person in Polish (my = mój, your = twój, our = nasz, your = wasz). Third person possessive pronouns (his, her, its, their) were absent. Since something was needed in their place, the genitive of regular personal pronouns was used instead (literally: his -> of him = jego; her -> of her = jej; their -> of them = ich). But we can still interpret them as possessive pronouns. In Polish, all pronouns should inflect for case, but since they are already inflected for genitive, it can't be done again. That's why jego, jej and ich always stay the same.
"In linguistics, declension is the inflection of nouns, pronouns, adjectives, and articles to indicate number (at least singular and plural), case (nominative or subjective, genitive or possessive, etc.), and gender. A declension is also a group of nouns that follow a particular pattern of inflection."
'to decline' is just a verb that means inflecting given word for these categories(number, gender, case).