In general, spin affects the direction in which the ball will travel after hitting the ground.
Side spin requires the use of special golf balls with spin ability.
With a special golf ball equipped, you'll be able to add side spin to your shot by tapping on the golf ball icon on the right as you're taking your shot.
This will bring up the ball spin mechanics where you can rotate the ball left or right to add side spin.
Shots with side spin will propel the ball left or right depending on the direction of the spin.