Learn Spanish (Spain): La tostada está fría.

We use está (from the verb estar) because being cold is a temporary state for this toast. If we used es, it would mean the toast is inherently a cold thing, like ice cream! Since toast is usually warm, being cold is a condition, so we need está.

Why is it fría and not frío?

Adjectives in Spanish must match the noun they describe in gender and number. Because la tostada is a feminine and singular noun, the adjective must also be feminine and singular, giving us fría.

Why does the word fría have an accent mark on the letter i?

The accent mark breaks up what would normally be a single blended sound (called a diphthong). Without the accent, you would pronounce it all in one quick syllable. The accent tells you to emphasize the i and pronounce the vowels separately in two distinct syllables: frí-a.

In English, "toast" is usually uncountable and we have to say "a piece of toast." Is it the same in Spanish?

No, tostada is a perfectly countable noun in Spanish! You can say una tostada for one piece of toast, or dos tostadas for two pieces. So la tostada simply refers to the specific piece of toast you have.