Learn Hardcore Icelandic: Smjörið er kalt í dag.

Smjörið is smjör (butter) + the suffixed definite article -ið (neuter singular).
So smjörið means the butter (some specific butter you’re talking about, e.g., the butter on the counter).

How do I know smjör is neuter, and why does that matter in this sentence?

Smjör is a neuter noun in Icelandic. Gender matters because adjectives agree with the noun’s gender/number/case.
Since the subject is neuter singular, the adjective appears as kalt (neuter singular), not kaldur (masculine) or köld (feminine).

Why is the adjective kalt and not something like kaldið?

Kalt is an adjective in its indefinite form used predicatively after vera (to be). Icelandic normally does not add the definite article to adjectives in this kind of simple “X is Y” statement.
So: Smjörið er kalt = The butter is cold.

What are the other forms of kaldur (cold), and how would the sentence change with a different noun?