Learn Italian: Un'arancia è uno spuntino ottimo. - An orange is an excellent snack.

Because arancia is a feminine noun that starts with a vowel. Normally, the feminine word for "a" or "an" is una. When a feminine noun starts with a vowel, the final a is dropped to make pronunciation smoother, and we replace it with an apostrophe: una + arancia = un'arancia.

Why do we use uno for spuntino instead of just un?

Because spuntino is a masculine noun that starts with an "s" followed by a consonant (in this case, "p"). In Italian, masculine words starting with this "s + consonant" combination always take uno instead of un to make the sentence flow better.

Would I also use the apostrophe for a masculine word starting with a vowel?

No! The apostrophe is only used for feminine words starting with a vowel (where una shrinks to un'). For masculine words starting with a vowel, you just use the normal un without an apostrophe. Because of this rule, seeing un' tells you right away that the noun is feminine.

Why does ottimo come after spuntino?