From the maintainer's site:

DROP (Don't Route Or Peer) and spamhaus_edrop are advisory "drop all traffic" lists, consisting of netblocks that are "hijacked" or leased by professional spam or cyber-crime operations (used for dissemination of malware, trojan downloaders, botnet controllers). The DROP and spamhaus_edrop lists are a tiny subset of the SBL, designed for use by firewalls and routing equipment to filter out the malicious traffic from these netblocks.

The DROP list will not include any IP address space under the control of any legitimate network - even if being used by "the spammers from hell". DROP will only include netblocks allocated directly by an established Regional Internet Registry (RIR) or National Internet Registry (NIR) such as ARIN, RIPE, AFRINIC, APNIC, LACNIC or KRNIC or direct RIR allocations.

When implemented at a network or ISP's 'core routers', DROP and spamhaus_edrop will help protect the network's users from spamming, scanning, harvesting, DNS-hijacking and DDoS attacks originating on rogue netblocks.

Spamhaus strongly encourages the use of DROP and spamhaus_edrop by tier-1s and backbones.

There is also a FAQ for the DROP list here.