To my knowledge, the Silver Age of comics is defined by the period of time from the mid-50s to the early 70s when comic books were seeing a resurgence in popularity. Was it due to the fact that comic art was getting better? That the stories were getting more deep and meaningful? Well, that's a debate for some other time.
It was called this because it was seen as a second great age of comic books (the first being the Golden Age - a period from the 30s to the 40's - in which comic books were new, exciting, and popular.)