I've been collecting comics for twenty years now. I love the hobby and would go to the local comics store regularly picking up weekly/monthly issues from my pull list. As I progressed further in the hobby, I started to make more friends that started collecting “key” comic issues.
What we mean by the term “key” is that the issue is significantly important in that it might contain an event that can shock a storyline prompting collectors to collect those comics. Events such as Secret Wars, Civil War, The Infinity Gauntlet, Death of Superman and more recently the death of Wolverine are some examples. These are key events and collectors will scramble to get the best copies, variants and also aim for the best possible “condition” of the book available.