"Briogais dhearg."

Translation: Red trousers.

[DebF26]

I'm very confused about when the second letter of a word we've seen (Dearg) becomes H (Dhearg) and why.

[silmeth]

That’s called lenition and what’s causing it is explained in the tips and notes to the lessons whenever it appears.

You can find the tips and notes in the web browser version of Duolingo (at the https://duolingo.com website), and also on the duome.eu website. Unfortunately they might not be available in the mobile app, so if you’re on a mobile device, you might want to open your web browser for the reading.

(And in this particular case the lenition is caused by the fact that the noun briogais that the adjective is attributing is feminine; and feminine nouns cause lenition of their adjectives.)