how to lock a folder on macbook air

If it is in your directory then a user in another directory cannot get access to it unless you have set permissions to let them.

You can't password protect a folder. The nearest you can do is set up an encrypted sparse image in Disk Utility which creates a flexible size "drive" in which you can store items and requires a password (do not forget it or all is lost!). See: https://www.intego.com/mac-security-blog/how-to-use-apples-built-in-features-to-encrypt-files-and-folders/

Select the folder in Finder. Right Click on it --> Get Info --> locked button