Complex sentence: LMDE has a semi-rolling release (partially rolling) development model; the difference is that, unlike Debian Testing constantly receiving updates, Linux Mint periodically introduces "Update Packs" which are tested snapshots of Debian Testing.
Split into simpler sentences: LMDE has a semi-rolling release (partially rolling) development model. This means that, unlike Debian Testing (a "real" rolling release) which constantly receives updates, LMDE periodically introduces "Update Packs" which are tested snapshots of Debian Testing.