Learn Italian: È uno zaino pesante. - It is a heavy backpack.

Because zaino is a masculine noun starting with the letter 'z'. Italian uses the special article uno (instead of the usual un) for masculine words that start with 'z', as well as 's' followed by another consonant (like uno studente). This makes the words flow better when spoken.

I see that pesante (heavy) comes after zaino (backpack). Is this normal?

Yes! In Italian, adjectives (describing words) usually come right after the noun they are describing. So instead of 'a heavy backpack' like in English, you say 'a backpack heavy'.

Zaino is a masculine word ending in -o. Why doesn't the adjective end in -o too, like pesanto?

Not all adjectives change their endings to -o or -a. Pesante belongs to a common group of adjectives that naturally end in -e for singular words. They use this exact same -e ending whether the noun is masculine or feminine.

Where is the word for 'it' in this sentence?

Italian frequently drops subject pronouns (like I, you, he, she, it) because the verb already gives us enough clues. The word È literally just means 'is', but it functions as 'It is' here since the subject is understood.