Learn Russian: Заткнись и слушай!

Заткнись is perfective because it demands a single, immediate change of state (close your mouth). Слушай is imperfective because it asks for a continuous, ongoing action (keep paying attention). Mixing aspects in commands is very common in Russian when you want an immediate action followed by an ongoing one.

Where does the word заткнись come from?

It is the reflexive imperative form of заткнуть, which literally means "to plug" or "to stop up" (like putting a cork in a bottle). So заткнись literally means "plug yourself." It is highly colloquial and rude, exactly like "shut up" in English.

Why does the reflexive ending change to -сь instead of -ся here?

The reflexive particle -ся shortens to -сь whenever it directly follows a vowel. In the imperative заткнись, the letter before the particle is the vowel и, so we must use -сь. If you were talking to multiple people, the plural imperative would be заткнитесь (because е is also a vowel).