Learn Italian: Sono d'accordo su questo punto.

English uses the action verb "to agree," but Italian uses the phrase essere d'accordo, which literally means "to be of agreement." Since sono means "I am," the phrase sono d'accordo is how you say "I agree."

Why is there an apostrophe in d'accordo?

It is a contraction of the preposition di (of) and the noun accordo (agreement). When di is followed by a word starting with a vowel, the "i" is dropped and replaced with an apostrophe to make it easier to pronounce.

Why use the preposition su here?

Su means "on" or "about." Just like in English where you say you agree "on" a point or "about" a topic, Italian uses su after essere d'accordo to introduce the topic you are agreeing about.

Could sono also mean "they agree" here?

Yes! The verb form sono is used for both "I am" and "they are." Without extra context or a subject pronoun like io (I) or loro (they) at the beginning, this sentence could just as easily mean "They agree on this point."