Learn Icelandic: Þetta eru góðar kökur.

The singular word for cake is kaka. It is a feminine noun. Feminine nouns that end in -a in the singular almost always change that -a to -ur to make the bare plural. Since this lesson is about bare nouns, kökur simply means "cakes" rather than "the cakes".

Why does the sentence start with Þetta (this) instead of a plural word for "these"?

When pointing something out with the phrase "This is" or "These are", Icelandic always uses the fixed neuter singular word þetta. It doesn't change to match the plural noun! However, the verb does change to plural, so Þetta eru... literally translates to "This are...".

Why does the adjective take the form góðar?

Adjectives must match the noun they describe in gender and number. Because kökur is a feminine plural noun, the base adjective góð (good) adds the feminine plural ending -ar to become góðar.