An adjective is a word that describes or modifies a noun (an interesting book, an American businessman). In English, all adjectives come before the noun that they describe. This is not the case in Italian. In Italian, most adjectives follow the noun that they modify, but there is a category of commonly used adjectives that precede the nouns.
That is, if a noun is masculine plural (fratelli [brothers], for example), the adjective that modifies it must be masculine and plural as well (fratelli maggiori [older brothers]). This means that there can be up to four forms of each adjective: masculine singular, feminine singular, masculine plural, and feminine plural.