They are the same action, but with different intentions. 

Covering fire is when you're trying to draw enemy fire away from something, such as your medic trying to stabilize somebody and move them to cover. 

Supressing fire is for when you want the enemy to have their heads down. No peeking at your movements, shooting back, or performing any movements themselves. 

I am a POG though so maybe I'm wrong. 🤷🏻‍♂️