How does friction affect Newton's second law?
Newton's 2nd law states acceleration is proportional to the net force acting on an object. The net force is the vector sum of all the forces applied to the object. It is basically described as the unbalanced force.
Friction always acts in the direction opposing motion. This means if friction is present, it counteracts and cancels some of the force causing the motion (if the object is being accelerated). That means a reduced net force and a smaller acceleration.
In the case of an object already in motion (like a puck sliding on an ice surface) the force of friction is the unbalanced force. In this case the acceleration (slowing down) of the puck is proportional to the amount of friction. More friction and the puck slows to a stop faster.