Learn French: Ce poisson est frais.

Poisson is a masculine singular noun that starts with a consonant, so we use ce. We use cette for feminine nouns, and cet for masculine nouns that start with a vowel sound.

Can ce poisson also mean 'that fish'?

Yes! French demonstrative adjectives like ce don't distinguish between 'this' and 'that'. It translates to either depending on context. If you need to be specific, you can add a suffix and say ce poisson-ci (this fish here) or ce poisson-là (that fish there).

Since frais ends in an 's', is it plural?

No, frais is singular here because we are talking about one fish. Some French adjectives naturally end in an s in their base singular form. When you make it masculine plural, it stays exactly the same: ces poissons sont frais.

How would this sentence change if the noun was feminine, like viande (meat)?

We would use the feminine demonstrative adjective cette, and the adjective would need to be feminine too. The feminine form of frais is irregular: fraîche. So it would become Cette viande est fraîche.