Dave_L wrote:
"prune" basically means deleting. When you prune a tree, you cut off the superfluous branches. In the context of a forum, pruning means deleting unwanted topics or posts.

A "digest" is a summary. A digest of the forum posts for the last week might be a single email containing the titles of the posts, or it might be a single email containing the complete posts.

"digested" and "un-digested" sound like odd made-up words, but I guess they refer to a digest as I just described.