Hammer follow is when the hammer will not remain in single action after the cycling of the slide. As the slide comes forward, the hammer goes into half cocked or in some cases will let down completely. Essentially, it turns the gun into a sort of an automatic gun. It can be dangerous if someone is not expecting multiple shots with a single press of the trigger. As for the springs, the higher rated ones will add more pressure to the sear, and ultimately a stronger sear/hammer engagement, which helps to prevent hammer follow.
I hope that made sense.
