Hi, this is a great question!

I'm glad that you understand the concept of pruning nicely, perhaps maybe I should provide an example in the article.

Suppose we have two frequent sets {A, B}, {B, C}, and an infrequent set {A, C}.

You see, {A, B, C} will be a candidate set since we could get it by joining {A, B}, {B, C}. However, {A, B, C} contains an infrequent set {A, C}. Therefore, this candidate set will be pruned, and we don't even bother to calculate the support of it!