Do I need a separate password for my MacBook Air and iPhone?

It depends on how you set them up. Usually only one Apple ID is used per person (and there is only one password per Apple ID). I use a shorter (4 or 6 digit) Lock Screen passcode on my iPhone and for my user login on my Mac. They can be the same on both devices or different.

Note that if you want any data to be able to sync via iCloud between the 2 devices, you need to use the same Apple ID on both.

You can have the same or different login passwords for each device. The only password which is constant is your Apple ID linked to those devices..