To do something that helps, aids, or benefits another person. If you do someone a good turn, they are much more likely to help you out in the future.

What does it mean to do a good turn?

a helpful and friendly act; good deed; favour.

What is a good turn example?

Your Scouts or Venturers might decide that a Good Turn could be something as simple as: Holding open a door for someone who has their hands full. Asking someone who might be having a bad day if they want to talk. Picking up a piece of trash that someone else left behind.