Am I able to trade in a iPhone that’s locked?

• Locked? do you mean disabled because you forgot the passcode? Remove the passcode:--> If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support
• Locked to a specific carrier? Contact that carrier and have it unlocked, if you're eligible.
• Activation lock? No way to remove that lock.

If you mean something different, please better explain.

Regards

Giulio

How is it locked exactly?

You won't be able to trade it in if its activation locked or locked with a passcode. you need to be able to restore it to factory condition to trade it in.

If its locked to particular carrier, than that is not an issue.