What is a growth mindset? Well what better way to explain this than show you with this picture?
So it's a process of keeping your mind "open" and therefore more likely to grow.
This site has some excellent information in regards to growth mindset.
This article shows growth research on children who undertook their SATS
I first heard about growth mind set from Class Dojo and they have created some awesome resources and videos explaining in a child friendly way. These clips are worth watching:
http://www.bbc.co.uk/cbeebies/grownups/help-your-child-try-new-things
So how can you apply this to your players on your session on a Sunday morning?
Use the growth mindset as a great model for developing people not players! As the players get older in my experience, players (and parents) get very focussed on what position they play. Use the principles of play and more importantly judge your players development and progress with the amount of smiles you see.